Battle of Britain
The Battle of Britain refers to the World War II German air campaign against Britain, in 1940 when Germany attempted, and failed, to gain air superiority in advance of their proposed invasion of Britain, Operation Sea Lion. Although the invasion was subsequently called off, mainly because of their defeat in the Battle of Britain, German air attacks continued throughout the war, including later use of the first cruise missile, the V-1, and ballistic missile, the V-2.
The Battle proper had the primary focus of supporting the invasion, but it was hoped that it, as well as later bombing and missile attacks on Britain, would break civilian morale. In retrospect, while attacks on both British and German populations caused much misery, morale never broke.
By June 1940, Britain was the only country still at war against Nazi Germany. The British Army had been repatriated to Britain in the Dunkirk evacuation, but had lost much of its equipment, and was in no state to match the German Army. Britain, and its leader Winston Churchill were braced for invasion:
The Battle of France is over. The Battle of Britain is about to begin. Upon this battle depends the survival of Christian civilization… The whole fury and might of the enemy must very soon be turned on us. Hitler knows that he will have to break us in this island or lose the war… Let us therefore brace ourselves to our duty, and so bear ourselves that, if the British Empire and its Commonwealth lasts for a thousand years, men will still say, 'This was their finest hour'. Winston Churchill, speaking in the House of Commons, 11 June 1940.
In the path of invasion lay the English Channel, and to take an army across that required air supremacy. Thus the Battle of Britain that ensued, became a battle for control of the skies over the Channel, between the Royal Air Force (RAF) and the Luftwaffe.
"The destruction or paralysis of the Fighter Command was ... an essential prerequisite to the invasion of these Islands." Air Chief Marshal Hugh Dowding 
Bombers were the main German weapon, opposed by British defenses that seemed, to the Germans, to mainly consist of fighter aircraft, with anti-aircraft artillery in a very secondary role. The Germans recognized that the British fighters were under ground control, but were unaware that they were attacking what, in modern terms, would be called an integrated air defense system (IADS). They came close to disabling it, but gave up and switched to a countervalue targeting of bombing cities. They failed to understand that the IADS was a center of gravity on which success or failure depended.
Different sources agree that the battle had four phases, although there are minor disagreements about their exact dates:
- Armed reconnaissance against coastal shipping
- Attacks against inland airfields
- Concentrated attack against London, but not the entire IADS controlling the fighters
- Harassing fighter-bomber attacks
By the end of the battle, the Germans concluded that an invasion was not possible. Not all bombing of Britain stopped with the end of the battle, although after the invasion was cancelled, the air battles were less intense and there were few attacks on the British IADS. What did change, however, is that the German night bombing attacks used electronic navigational aids, and British electronic intelligence and electronic warfare, especially meaconing became what Churchill later called "the Wizard War".
German doctrinal and political background
Luftwaffendienstvorschrift 16: Luftkriegsführung (Luftwaffe Service Regulation l6: Conduct of the Air War) laid down three points:
- Subjugation of the enemy air force in order to achieve and maintain air superiority; this would include not only air combat, but attacks on air force facilities on the ground, and the means of producing aircraft and supplies for them
- Support of the army and navy
- Attack against the enemy infrastructure: transportation, industry, government and military administration, food production
Air defense planning
The German Air Force had responsibility for anti-aircraft artillery, which they considered would be adequate for defense of Germany. Defensive fighters were not seen as important; their role was to defend mobile military forces: bombers, ships, and ground troops. On these assumptions, the German aircraft industry planned to produce three bombers for every fighter. 
Strike warfare planning
Germany had made effective use of fighter-bombers and light bombers in close air support, and bombing cities, such as Rotterdam, without an effective air defense. As mentioned above, they did not see fighters as a significant means of defense, even though they embraced many other concepts from the Italian theoriest of the 1930s, Giulio Douhet, who saw air supremacy to be the first stage of any war. At the time he wrote, however, there was no real concept of an integrated air defense system.  Douhet believed the bomber could fight its way to and from the target, hence
the bomber will always get through. — Stanley Baldwin, House of Commons, 1932
Douhet theorized that population bombing would break morale and cause civilians to demand that their government sue for peace, while air attacks on industry would simultaneously cripple the military. The Germans did not see air power alone as forcing Britain to surrender, for which they saw physical invasion as necessary. They did not, however, have an evolved doctrine or forces for amphibious warfare, and their concept required total protection of the invasion force from the Royal Air Force and Royal Navy.
Germany also had not concentrated on the formation of a day-and-night capable strategic bombing force. In particular, while the British trained their crews in navigating using stars as references, which let them find city-sized targets at night, German crews did not have such training. Instead, the Germans had planned on getting their night bombers onto target using ground-based electronic navigational aids, for which the bombers had receivers.
Douhet overestimated the problem of destroying the enemy's air force (i.e., offensive counter-air).
There is no practical way to prevent the enemy from attacking us with his air force except to destroy his air power before he has a chance to strike us . . . . We must therefore resign ourselves to the offensives the enemy inflicts upon us, while striving to use all our resources to inflict even heavier ones upon him.
Through effective attacks on enemy airfields and aircraft factories, command of the air will be achieved quickly. 
Air Chief Marshal Hugh Dowding, commanding RAF Fighter Command, observed that the Germans, once they had established operational practices faced with the reality of British defenses, were sending four fighters for every bomber. Consider that in respect to the production rates that German policy had set, of more bombers than fighters, and a serious constraint on German endurance emerged. Hitler had an obsession with bombers as the focus of the German aircraft industry. When the revolutionary jet-propelled Me-262 first flew, as a fighter, Hitler was enraged, and demanded that it be given bombing capability. The resultant delay in making it available as a fighter prevented its devastating use against Allied bombers later attacking Germany; there was no real countermeasure against it other than destroying its bases and factories.
In October 1939, Dowding objected to the desire of the Chief of Air Staff, Sir Cyril Newall, to plan to send fighter reinforcements to the Battle of France. A total of ten squadrons were sent, from a total of thirty-nine. The Air Staff had estimated sixty squadrons would be needed for the defense of Britain. 
Dowding wrote that the Germans enjoyed the initiative, and exercised it with raids from different directions, some of which were feints. "First a blow would be delivered from Calais, perhaps against London; then after a carefully-timed interval, when 11 Group Fighters might be expected to be at the end of their petrol endurance, a heavy attack would be made on Southampton and Portland. Other attacks, after being built up to formidable dimensions, would prove to be only feints, and the bombers would turn away before reaching coast of England, only to return again in half an hour, when the fighters, sent up to intercept them, were landing."
The Germans continued to adjust their fighter tactics, starting with having the fighters fly above and behind the bombers. The mile of separation involved allowed the British to attack the bombers before the fighters could engage the interceptors. By September, "fighter escorts were flying inside the bomber formation, others were below, and a series of fighters stretched upwards to 30,000 feet or more. One Squadron Leader described one of these raids as "like looking up the escalator at Piccadilly Circus". "
In modern terms, Britain had an integrated air defense system, with a significant degree of tolerance to damage to individual sensor or control stations. Fighter Command Headquarters had the "big picture", which, in turn, went to Group level, and then to the key operational command centers, Sector Stations at airfields.
Radar warning information went directly to Fighter Command, which distributed the information to Groups and Sectors.
The original radar, called Chain Home, had an effective range of 80 miles, and up to 200 miles under favorable conditions. The Chain Home stations, which were spaced approximately 30 miles along the coast, could not detect aircraft flying below 1000 feet, and so another system, Chain Home Low, was installed to detect low-flying aircraft, especially those dropping naval mines. Chain Home Low had a 30 mile range. 
Three of the radio-location stations in the south of England suffered severe damage and casualties. No station was permanently put out of action, and the worst damage was repaired in about a month, though the station was working at reduced efficiency in about half that time. The operating personnel, and particularly the women, behaved with great courage under threat of attack and actual bombardment.
There were also the Royal Observer Corps, who reported visual and audible sightings of German bombers. Their reports went to their local sector, which distributed it to Fighter Command and other relevant Sectors.Dowding credits the commander of the Royal Observer Corps, Air Commodore A. D. Warrington Morris. Dowding emphasized that they were the only means of tracking raids once they had passed the coast, and, at night, only sound location, and some searchlight spottings. These were adequate for air raid warning, but not for interception.
A limited solution to inland tracking came when Dowding obtained a short-range (40,000 feet) "G.L." Army gun control radar. 10 sets were made available, as an experiment, "in the Kenley Sector on the usual line of approach of London raiders, which commonly made their landfall near Beachy Head. The G.L. sets were installed at searchlight posts, and direct telephone communication was arranged with the Kenley Sector Operations Room." 
Radar reports did not go directly to the tactical plots, but first to a "filter room", where increasingly experienced operators eliminated false tracks, and then announced them over a telephone line connected to Fighter Command Operations, as well as all Groups and Sectors concerned. 
Interception became enormously more difficult at night. As Dowding put it, "it became necessary to put the fighter within one or two hundred yards of the enemy, and on the same course, instead of the four or five miles which were adequate against an illuminated target." Searchlights had been effective for this in the First World War, but they were generally defeated because the bombers flew high and fast, sufficiently so that speed-of-sound delay was a significant factor for the audio locators that guided the searchlights. Only airborne radar could give the necessary speed and precision of location, unless the searchlights could be radar-directed.
Sectors were the key element of controlling the three defensive systems: interceptors, anti-aircraft artillery (AAA), and barrage balloons. Once the direction of a raid was known, barrage balloons along its path were raised, to force the bombers to higher altitudes. The higher the altitude of the bombers, the less accurate were the bombsights, and the more time fighters had to intercept it while the bombers were climbing.
Sector controllers would alert AAA when the bombers were coming into range. The technology of the time used time-fuzed antiaircraft shells that would burst at a predefined height, and, with the plotting information from the sector controllers, as well as locally controlled searchlights, the artillery would try to put the bursts into the bomber streams. The sector controllers also avoided having antiaircraft firing into the same area where fighters were operating, avoiding fratricide.
Fighters were kept at several readiness levels:
- Standby (+2 minutes to launch)
- Readiness (+5 minutes to launch)
- Available (+20 minutes to launch)
If a raid was especially heavy, Fighter Command might order fighters from other Groups into the threatened area, usually that of Group 11 covering London, the invasion areas, and the coastal radars and sector stations.
"As regards aerodromes, Manston was the worst sufferer at this stage. It, Hawkinge and Lympne were the three advanced grounds on which we relied for filling up tanks when a maximum range was required for operations over France. They were so heavily attacked with bombs and machine guns that they were temporarily abandoned. This is not to say that they could not have been used if the need had been urgent, but, for interception at or about our own coastline, aerodromes and satellites farther inland were quite effective. 
After the sector controllers had decided to launch fighters, the fighters had to be directed to the bomber formations. The primary means used by the sector controllers was radio, both to converse with the fighters, but also, with separate direction-finding antennas and receivers, to track the fighter formation continuously after takeoff. To avoid German interference with fighter-to-ground communications, the radios involved were equipped with identification-friend-or-foe systems codenamed "Parrots", and a ground controller could order fighters to "squawk their parrots" for positive identification. 
The air raid warning system
Operated centrally by Fighter Command, with some minor exceptions, this system split the country into 130 "Warning Districts". A warning table, separate from the operations display, kept track of all raids, with color-coded counters. The colors corresponded to a 5-minute interval along the track, so "stale" counters could be removed. 
The districts were defined by the structure of telephone exchanges, through which warnings were disseminated. Three operators at Headquarters were linked continuously to counterparts at the regional telephone centers in London, Liverpool and Glasgow. When a raid was within 20 minutes' flying time of a district, the air raid warning controller would send a message to the appropriate region, such as "10. Norwich. Yellow." This would go to the nearest regional operator in London, who would pass it to Norfolk, who would then alert the emergency services; no public warning was sounded as yet.
If the raid remained on track for Norwich, a "Red" signal would be sent 5 minutes later, and the "air raid" sirens went off. Should the raid pass the target, a "green" signal would be telephoned and the "all clear" sirens would sound.
A special case, for military facilities that had to be working at night, an additional, non-public "Purple" signals was sent to such targets, telling them to turn off exposed lights.
At night, when it became essential to maintain exposed lights in dockyards, railway sidings and factories up to the last minute, so as to obviate unnecessary loss of working time, a "Purple" warning was introduced. . This was a signal for the extinction of exposed lights, but it did not connote a public warning.
As part of the electronic warfare program of denying navigational aids to the Germans, a fourth operator would order radio stations, on the projected path, so they could not be used as directional beacons.
As early as 1939, British scientific intelligence had become aware of the German emphasis on radionavigation for bombing. The first clues were from the Oslo Report, a HUMINT report whose author, Hans Ferdinand Mayer, became known only after the war.  Other clues came from analyzing the radios on crashed German bombers, which seemed to have radios more sensitive that would be needed for communications alone.
"During the human struggle between the British and the German Air Forces, between pilot and pilot, between AAA batteries and aircraft, between ruthless bombing and fortitude of the British people, another conflict was going on, step by step, month by month. This was a secret war, whose battles were lost or won unknown to the public, and only with difficulty comprehended, even now, to those outside the small scientific circles concerned. Unless British science had proven superior to German, and unless its strange, sinister resources had been brought to bear in the struggle for survival, we might well have been defeated, and defeated, destroyed." — Winston Churchill
According to R.V. Jones, Britain's head of scientific intelligence (and, at one point, its only scientific intelligence officer),
In June, 1940, I received the culminating information on a trail that I had been following for some months, which led me to the conclusion that Germans had developed a radio beam system for blind bombing known as Knickebein. They had taken care to disguise the receiver in their aircraft as being designed for blind landing, but we managed to unmask its true purpose. The importance of this development was great. The evacuation from Dunkirk had just occurred, our air defences, while excellent by day, were almost impotent by night, and the whole of the German bomber force could come over then and drop most of its bombs into the area of intersection of the beams, which were little more than half a mile wide over London.
Jones said that Prime Minister Winston Churchill and his scientific advisor, Lord Cherwell, made RAF resources available to confirm Jones' hypothesis. The RAF created the first airborne electronic intelligence unit, equipped with three obsolescent Avro Ansons fitted with suitable radio receivers. . They detected directional beams coming from stations on the French coast. On 21 June, the existence of the Knickebein radio navigation system was confirmed. 
British counterintelligence was also able to deceive the Germans as to where their bombs, directed by radionavigation, were landing. With the Double-cross system, the U.K. had captured every German agent sent into Britain. Some were jailed or executed, but a sufficient number were doubled such that the British could use them in systematic deception campaign against the Germans. For example, the double agents reported what they claimed to be the impact area of bomber (and, later, missile) raids, but the sequence of reports systematically shifted the points in a safer direction than the actual impact point. 
Battle of France, Dunkirk evacuation, and BoB
After the "Phony War" following the fall of Poland, Germany turned the Western Front into very real war on 10 May 1940. From the air standpoint, the Germans were flying within the doctrine and organization of blitzkrieg, while the Allies had relatively few modern fighters, and no coherent doctrine for air defense or even close air support.
"Fighting the last war" is a complaint about many military leaders, but the Luftwaffe was gaining confidence because it was successfully refighting the Polish campaign, against defenses that did not form an integrated system. The defenses of Britain itself, however, were quite different.
As the French fell back, Churchill met with the French leadership.
there was a considerable silence. I then asked: "Where is the strategic reserve?" and, breaking into French, … "Où est la masse de manoeuvre?" General Gamelin turned to me and, with a shake of the head and a shrug, said: "Aucune." [none]
At that point, Churchill made the hard strategic choice that France was lost, and the British fighters were needed both to cover the evacuation and for the defense of Britain.
Dowding, who had been opposed to releasing fighters to the Battle of France, still was reluctant to risk the defense of Britain even to protect Dunkirk. RAF fighters providing cover over the Dunkirk background were not a separate force, but part of No. 11 Group. They flew more than 3,000 sorties in the nine days, losing 959 while shooting down over 1200 German aircraft. 
There are many theories on why the Germans allowed the Dunkirk evacuation, or if they truly were unable to stop it. One account was that Hermann Goering had insisted that the Luftwaffe have the honor, and Adolf Hitler ordered the German armored forces to hold their positions. Dunkirk, nevertheless, was an extremely close thing, and the loss of 100 British fighters and 80 pilots seemed nearly irrecoverable.
By 18 June, all British forces had withdrawn from France. Both the German Air Force (Luftwaffe) and the RAF had lost many aircraft and trained crews during this campaign. Several weeks passed while the Luftwaffe replaced their losses and took over airfields in the countries they had captured. In Britain the time was spent putting as many new fighters and trained pilots into service as possible, to guard against the attack everyone knew was coming. The lull as the German forces consolidated their position was vital to the British armed forces, as it allowed them to prepare.
The RAF readies
By the beginning of July 1940, the RAF had built up its strength to 640 fighters, but the Luftwaffe had 2600 bombers and fighters. The stage was set. In the skies above South East England, the future of Britain was about to be decided. As the Prime Minister, Winston Churchill put it; "What General Weygand called the Battle of France is over, the Battle of Britain is about to begin".
Britain was still urgently increasing fighter production. Dowding and Minister of Aircraft Production, Lord Beaverbrook, while opposites in personality, worked together extremely well. Beaverbrook almost doubled production; fighters came off the production line and went immediately to operational airfields. Beaverbrook also developed effective means for salvaging wrecked British aircraft.
Command structure & order of battle
Neither side had an ideal command system.  There were doctrinal disagreements between the commanders, and the national leadership, on both sides, took action for political rather than strictly military factors.
Britain had better command and control, and a simpler challenge; a bomber shot down was a victory to the British, while the Germans were concerned not only if the bomber survived, but if it hit the target. Britain also had the advantage of fighting over its own territory, which meant that pilots able to parachute from a dying aircraft could be back in service quickly. It took longer to train a pilot than to build a fighter.
The British had to survive, while the Germans had two requirements: survive, and create the necessary conditions for the invasion. Defining the latter, however, was frustrating, because the German Navy, Army and Air Force all had different assumptions over the needs of the invasion and the extent to which British air had to be suppressed.
Royal Air Force
Winston Churchill, of course, was Prime Minister and Minister of Defense, with ultimate authority. In overall command were Air Chief Marshal Sir Cyril Newall, Chief of the Air Staff, and Air Chief Marshal Sir Hugh Dowding, Air Officer Commanding-in-Chief of Fighter Command. Dowding presented his plans to the War Cabinet on May 15, after discussions among the senior chiefs. While Dowding was known to Churchill, they were not close. Coming into the meeting, Dowding, nicknamed "Stuffy" for his stubborn and argumentative personality, had several grievances. He believed he should have been Chief of Air Staff, he believed the Air Staff had been ignoring Fighter Command, and the Staff had told him that he was almost certainly to be retired when his appointment ended on July 14. Dowding however was intent to make the defense requirements clear to Churchill. 
At the time of the Battle of Britain, Fighter Command was split into four groups:
- 10 Group (South West England), Air Vice-Marshal Sir Quintin Brand
- 11 Group (South East and London; the likely invasion area) Air Vice-Marshal Sir Keith Park
- 12 Group (the Midlands and Wales)Air Vice-Marshal Sir Trafford Leigh-Mallory
- 13 Group (North of England and Scotland)Air Vice-Marshal Richard Saul
Park and Leigh-Mallory had different ideas on how to fight the German bombers, and emotions got in the way of realizing that both were correct on the best way to counter bombers coming into their area of operations. 11 Group had to respond quickly, while 12 Group had more warning time, and thus the opportunity to organize larger fighter formations -- the "Big Wing", pushed by fighter leaders such as Douglas Bader.
11 Group fighters, close to the German bases, had no choice but respond to a warning with whatever was available. 12 Group, being further north, had more time for large numbers of fighters to take off from multiple airfields and assemble into the "big wing" advocated by Douglas Bader, a distinguished fighter pilot in 12 Group.
Dowding recognized that the individual groups had different problems and opportunities, and delegated detailed tactical decisions to the group commanders. While the Principle of Mass has been a historical maxim of war, technology and tempo of operations, both significant factors in the Battle of Britain, were modifying the assumption.  11 Group did not have the luxury of mass. Had they not had radar warning, since they did not have enough aircraft to keep an adequate force continually airborne, they would have been completely ineffective.
On the British side, the Defiant and Blenheim fighters were designs that worked in specialized roles such as long-duration patrols and night radar intercept, but that were almost helpless against single-engine fighters such as the German Me-109.
The delays in production, which resulted in only three aircraft being delivered before the outbreak of war, meant that it could not be used in 1940 in its originally planned role-that of standing defensive patrols and was forced into action as an interceptor alongside the Supermarine Spitfire and Hawker Hurricane.
The Boulton-Paul Defiant first entered combat in May, and achieved a somewhat patchy record. Against bombers, the Defiant's extremely heavy turret armament was very effective; and by operating mixed formations of Defiants and Hurricanes the RAF could make use of the superficial resemblance between the two types to confuse and trap German fighters. However, when the Messerschmitts caught Defiants on their own, and recognised them for what they were, they inflicted appalling casualties. The technique of the pilot positioning his aircraft with the gunner's field of fire in mind was feasible against slow bombers but quite impossible in a fast dogfight. Aircrew losses were high in Nos. 264 and 141 Squadrons; in an emergency a Defiant gunner had very little chance of escaping from his turret 
The Bristol Blenheim was ordered while still in design as a bomber, and, with British recognition of the nature of the immediate threat, 7 of the 16 squadrons in the RAF were converted to a fighter version with extra guns. "As a day fighter the Blenheim IF was a failure, proving to be fairly easy meat for single-engine interceptors, and casualties were high. Like the Defiant, it served as a useful night fighter where its roomy fuselage and comparatively high loiter capability could be utilised to the full in this role. 
- Chief of the Luftwaffe Reichmarshall Hermann Goering
- Generalfeldmarschall Albert Kesselring, Commander Luftflotte II, Northern France, Belgium and Holland (headquarters in Brussels)
- Generalfeldmarschall Hugo Sperrle, Commander Luftflotte III, rest of France with their Headquarters in Paris.
- Generalfeldmarschall Erhard Milch, Commander Luftflotte V from 12 April 1940 - 9 May 1940; Generaloberst Hans-Jürgen Stumpff, commanding from 10 May 1940 - 27 November 1943
There was limited involvement from Luftflotte (Air Fleet) No 5 was based in Norway, and from a few Italian units.
Hitler originally restricted attacks on cities, but changed that position over time. He was affected by the political perception created by Goering, who had promised that his defenses would prevent Berlin from being bombed.
The two co-equal air fleet commanders, Kesselring and Sperrle, differed on whether to continue attacks on air defense (Sperrle's position) or switch to city bombing (Kesselring's position).
In many respects, German amphibious warfare doctrine treated crossing the English Channel as a river-crossing problem on a larger scale. They expected to use flat-bottomed boats as landing craft, which would be loaded in French ports and would sail directly to their targets. The Germans did not use the Allied concept of lowering specialized landing craft from attack transport ships.
Small craft of this type would have needed ideal weather even to have attempted the invasion, but were vulnerable to being swamped by the waves produced by defending destroyers moving at high speed. Neither gunfire nor ramming would be needed to stop most of the invasion troops, as long as the Royal Navy was operational. The German army depended on the Luftwaffe providing complete protection against both the Royal Navy and Royal Air Force.
There seems to have been little co-operation between the German armed forces, and despite the impressive build up of barges and other equipment in the Channel ports, the actual detailed planning for the operation, code-named "Sea Lion", was never really thrashed out. All depended on the success of the Luftwaffe it would appear, before the invasion was to be taken seriously
One of the aircraft types used in these raids was the Junkers Ju-87 Stuka dive-bomber. These were very accurate, and had been particularly successful earlier in the war when there was no effective fighter opposition. Due to unacceptable losses against the first-line British Spitfire and Hurricane fighters, they were withdrawn from the battle in mid-August.
The Me-110 twin-engined fighter, intended as a "bomber destroyer", also proved to be vulnerable to attack by British single-engine fighters. Like the British Defiant and Blenheim, it later proved useful as a night fighter.
Phase 1, Kanalkampf, the Channel battles
From the standpoint of the Luftwaffe, operations in this period were essentially armed reconnaissance: "probing the British defences - looking for weaknesses before a major assault could be launched to exploit them.In modern terms, the German missions during this phase offensive counter-air (OCA) and suppression of enemy air defense (SEAD). They did not understand the significance of British radar and control systems. To invade Britain, the Germans had to have control in the air over the English Channel, otherwise the RAF and the Royal Navy would have been able to destroy their invasion force before it reached the shore.
Dowding's intention was not to stop German bombing, but to attrit its strength, especially the bombers. He did not want fighter-to-fighter combat, because German fighters could not severely hurt Britain. This was a reason he did not want to protect convoys in the Channel; he was still building up resources and he saw the short-notice combat as causing too many losses. 
During this time, the British were intensely researching the German radionavigation systems and countermeasures to them, although the Germans were not yet depending on them. By July, electronic intelligence receivers were present at a several coastal radar sites. These receivers allowed the British to learn the radionavigation frequencies that would be used that day.
In this phase, the major German attacks were on shipping convoys and on fighter patrols, the latter being met on terms advantageous to the Germans. They had an idea the radars were important, and attacked some of the radar antennas on the south coast. While they did damage antennas and associated electronics, these were relatively hard targets. A critical error was that they did not understand the significance of the sector control stations. Antennae were relatively easy to repair, but destroying a control station, with its highly trained staff at the center of a communications network, would have been vastly more significant.
Phase 2 (August 8-September 6), Adlerangriff, "Eagle Attack"
Personalities and emotions clouded German judgment and made them break off, prematurely, their attacks against the coastal defenses, not realizing the critical role of the Chain Home radar stations.  While "Hitler" and "restraint" are surprising to find in the same sentence, the reality had been that Hitler had forbidden the night bombers to hit London. Dowding called this phase
On 24 August, a small group of night bombers, probably having navigational errors and possibly lightening the load after suffering damage, flew over the edge of London, and at least one aircraft released its bombs. Given that Londoners were under strict orders not to show lights after dark -- the "blackout" -- it is even possible the bomber did not know it was over London. It is unclear when Britain started jamming the Knickebein system; Kopp writes that 15 "Aspirin" jammers were in operation by October, and if the night bombers might have made their error due to electronic countermeasures.
Churchill responded with a "retaliatory raid" against Berlin, doing relatively light physical damage but having an immense psychological effect, as did the subsequent U.S. Doolittle raid against Japan. Hermann Goering, commanding the Luftwaffe, which included the anti-aircraft artillery, had made public boasts that "if one British bomb falls on Berlin, you can call me Meyer [a common and somewhat derogatory German name]."
Even after this, Hitler still did not immediately order attacks against British cities. German intelligence was also extremely poor. Every German spy in Britain had been neutralized Double-Cross system, and British counterintelligence was sending deceptive material. As far as known, the Germans did not realize that by early September, the air defense system was largely exhausted. With hindsight, had the Germans continued to concentrate on airfields and radar, they were close to defeating the British IADS.
Churchill knew this risk, and there are a number of reports that he sent two more missions over Berlin, on 28 August and 3 September, with a psychological goal: provoke the Germans into retaliating against cities, and give the IADS time to recover.
Phase 3, the Blitz (7 September-October 6)
Hitler did as Churchill had hoped, switching the attack to population and industry. The early bomber theorists believed, incorrectly, that civilian morale could be broken by attacks on housing, and that the technology of the time could reliably destroy industry. The German forgot, however, that Douhet had given the greatest priority to taking command of the air before going after industry:
“The one effective method of defending one's own territory from an offensive by air is to destroy the enemy's air power with the greatest possible speed.” — Giulio Douhet
They also hoped that RAF fighters would gather in force around the cities to protect them, which would make it easier for the Luftwaffe to shoot them down in the numbers required to establish air superiority. The change of plan was a mistake for several reasons. It gave 11 Group a chance to repair their airfields and radar sites, so the defences became fully operational again. The Me-109 fighter could only carry enough fuel for 20 minutes flight over Britain, so London was on the edge of its limited range. Finally, the German raids now came within the range of 12 Group, and their large formation tactics known as "Big Wings".
Knowing the target to be London and the industrial centers, the British controllers now had time to assemble a large number of fighters to attack the German formations and break them up before they could bomb. The appearance of large numbers of Hurricanes and Spitfires came as something as a shock to the Luftwaffe pilots, who had been told by their intelligence officers that Fighter Command had practically been wiped out by the earlier raids against the airfields. By changing tactics and targets, the Germans had actually helped Fighter Command to deal with raids.
For those living in the cities, the Blitz had begun, as night raids followed daytime raids and gave the civilians little rest. Increasingly, the bombing was at night, which also meant that any British ECM would be effective.
The British were having some success with early airborne radar, which the deprecated Blenheims and Defiants, as well as the new Beaufighters, were large enough to carry. Some attempts were made to use Hurricane and Spitfire aircraft at night, but any intercepts they made were from luck, ground radar guidance, or both.
On the 15th September the Germans delivered their maximum effort, when British guns and fighters together accounted for 185 aircraft. Heavy pressure was kept up till the 27th September, but, by the end of the month, it became apparent that the Germans could no longer face the bomber wastage which they had sustained, and the operations entered upon their fourth phase, in which some German fighters acted as bombers.
Phase 4: The End of the Battle, October 6-October 31
As the long, hot summer ran into October, the German daylight bomber losses became too heavy. Their bomber force started to operate only at night, and the damage they caused to Britain's cities was enormous. Many civilian organizations were set up to help deal with the wounded people and damaged buildings.
The German raids continued, with more electronic navigation assistance, as well as day raids by fighter-bombers. By now, British ECM was definitely in use, and the "Wizard War" was active.
These "Jabo", from the German for hunter-bomber, were more harass than a means of decision. Since the fighter-bombers flew high and fast, the radar warning that had been enough to alert fighters against slower bombers was inadequate. Only by flying airborne patrols could the RAF have any chance to intercept the Jabos, but the Jabos were also less dangerous than bombers with larger payloads.
Not with a bang, but a whimper
The gratitude of every home in our Island, in our Empire, and indeed throughout the world, except in the abodes of the guilty, goes out to the British airmen who, undaunted by odds, unwearied in their constant challenge and mortal danger, are turning the tide of the world war by their prowess and by their devotion. Never in the field of human conflict was so much owed by so many to so few, — Winston Churchill, August 1940
There was no clear-cut end to the Battle of Britain, but from a strategic standpoint, the Germans lost, because gaining air superiority was a prerequisite to the invasion. This was a costly and inefficient use of aircraft and pilots, exactly the situation the control system had helped to avoid during the earlier phases of the Battle, but German losses began to increase. The weather also began to worsen and the raids stopped in late October. Germany turned to electronically guided night bombing, countered by British electronic warfare in the Battle of the Beams.
The Germans then realized that the RAF could not be defeated in 1940, Operation Sea-Lion was cancelled indefinitely and eventually abandoned. The Battle of Britain, the first campaign decided in the air, did not so much end, as fade away. Raids continued, but Hitler's attention turned from Britain to the Soviet Union.
Dowding was retired on November 25. He had made enemies, and was, indeed, tired. The precipitating event was probably that he resisted stopgap changes that were hoped to stop the German bombing, which had moved to night attacks. That Britain did not have an adequate operational radar-equipped night interceptor was not relevant to the political pressures, which he indeed antagonized. 
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