Piracy has been a scourge of the seas for millennia, but its definition has become more complex with the evolution of international law. The current primary reference comes from the United Nations Convention on the Law of the Sea, Article 101: " Piracy consists of [inciting, performing, or facilitating]:
- any illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed:
- on the high seas, against another ship or aircraft, or against persons or property on board such ship or aircraft;
- against a ship, aircraft, persons or property in a place outside the jurisdiction of any State;
- any act of voluntary participation in the operation of a ship or of an aircraft with knowledge of facts making it a pirate ship or aircraft;
Actions against aircraft may be termed aircraft hijacking; U.S. and other national law defines specific crimes of air piracy.
Imperial Rome defined pirates simply, in the words of Marcus Tullius Cicero, as hostis humani generis, "enemies of the human race." That designation was an early example of universal jurisdiction; it was accepted that pirates were criminals that had put themselves beyond the law of nations, the only existing source of law. At the time, there was no distinction between territorial and international waters, and no source of international law. The idea of hostis humani generis grew to include other categories, such as slavery and genocide, but jurisdictional issues became more complex. There is considerable legal thought about bringing other offenses, such as transnational terrorism, under this doctrine, but that further complicates jurisdiction.
After the Roman Empire fell, pirates remained a real problem. The Viking raiders both operated as sea pirates, but also raided land.
Today, piracy at sea is a very real problem, but counter-piracy enforcement is complex.
Reprisal and Marque
While the usual phrasing of the framework for privateering is "letter of marque and reprisal", the idea of reprisal, or authorizing national merchant vessels to be armed against pirates, preceded the idea of marque, or using nationally authorized private warships used as part of foreign policy. Sir Francis Drake, for example, was a British privateer authorized by Elizabeth I, who still maintained diplomatic relations with Spain while Drake and others ravaged Spanish shipping and ports. In an early example of plausible deniability, she said of Drake, if he "shall at any time or times hereafter robbe or spoile by sea or by lance, or do any acte of unjust or unlawful hostilities [he shall] make full restitution, and satisfaction of all such injuries done." Of course, she subsequently knighted him. 
In an annex to the Declaration of Paris (1856), which ended the Crimean War, a number of nations agreed to stop issuing letters of marque and reprisal. By that time, standing national navies existed and were more controllable tools of national policy.
Modern jurisdictional issues
Article 101 of UNCLOS states "On the high seas, or in any other place outside the jurisdiction of any State, every State may seize a pirate ship or aircraft, or a ship or aircraft taken by piracy and under the control of pirates, and arrest the persons and seize the property on board. The courts of the State which carried out the seizure may decide upon the penalties to be imposed, and may also determine the action to be taken with regard to the ships, aircraft or property, subject to the rights of third parties acting in good faith." It does not, however, make it clear what authority States have against pirates operating within the territorial waters of another State, or on pirate bases ashore.
Current piracy and counter-piracy
Straits of Malacca
- See also: Somalia
In April 2009, one of the most active areas of piracy includes territorial waters of Somalia, as well as international waters in the area. Most of the pirates appear to be based in Somalia, widely accepted as the archetype of a failed state. Somalia’s interim president, Sheikh Sharif Sheikh Ahmed, a moderate Islamist called for international assistance, but said the problem was “is not on the sea, it’s on the land...only the Somali government can deal with those who are on the land.”  China, Libya, South Africa and Vietnam emphasized that their consent applied only to the Somali situation, and they considered UNCLOS still to be definitive.
The resolution endorsed the European Union's Operation Atalanta, with military forces from France, Britain, Germany and Greece and Spain. Its duties were described as escorting; it is unclear to what extent the force will pursue pirates and attempt rescue. French forces have recaptured at least two pirated vessels.
Several international counter-piracy forces are operating in international waters, but not under common command. In addition to Atalanta, Task Force 151 includes warships from the U.S., Turkey, and Singapore; Denmark has been part because it declined to join the EU force but did not want to undermine NATO.
Russian, Chinese, and Indian ships have conducted counter-piracy operations; an Indian warship set one alleged pirate mother ship on fire, which later was said to be a Thai ship seized by pirates.
Another area of modern piracy is in the waters off Nigeria.
- , Part VII, High Seas, United Nations Convention on the Law of the Sea (UNCLOS)
- Terrorism and Violent Crime Section, Criminal Resource Manual, Section 1406: Aircraft Piracy, Interference, and Other Title 49 Aircraft Offenses—Venue
- Burgess DR Jr (July-August 2005), "The Dread Pirate Bin Laden: How thinking of terrorists as pirates can help win the war on terror", Legal Affairs
- Roosz S, United Nations Security Council: Modern Maritime Piracy Update, Harvard Model Congress 2009
- McLure J (3 February 2009), "Somalia’s President Ahmed Pledges to End Piracy, Seek Peace.”", Bloomberg Press
- Security Council Condemns Acts of Piracy, Armed Robbery off Somalia's Coast, authorizes for six months "all necessary means" to repress such acts; Resolution 1816 (2008) Adopted Unanimously with Somalia’s Consent, United Nations Security Council
- Margaret Besheer (December 2, 2008), "UN Security Council Extends Anti-Piracy Measures off Somali Coast", Voice of America
- James Kraska (28 April 2009), The Report on the U.S. Naval War College Workshop on Somali Piracy: Fresh Thinking for an Old Threat