United States Northern Command
The United States Northern Command (USNORTHCOM) was created on 1 October 2002 to provide command and control of Department of Defense (DoD) homeland defense efforts and to coordinate defense support of civil authorities. It absorbed forces assigned to the U.S. components of North American Air Defense Command (NORAD), as well as various headquarters of units whose primary duty is operation within the United States; the commander of USNORTHCOM is also the NORAD commander. Its area of responsibility includes air, land and sea approaches and encompasses the continental United States, Alaska, Canada, Mexico and the surrounding water out to approximately 500 nautical miles. It also includes the Gulf of Mexico and the Straits of Florida. The commander of USNORTHCOM is responsible for theater security cooperation with Canada and Mexico. 
Not all U.S. states and territories are under USNORTHCOM. The defense of Hawaii and U.S. territories and possessions in the Pacific is under United States Pacific Command, while the defense of Puerto Rico and the U.S. Virgin Islands is the responsibility of United States Southern Command.
The Commander, US Northern Command (CDRUSNORTHCOM) is responsible for  providing defense support of civil authorities, as directed by the National Command Authority, and, in coordination with United States Strategic Command (USSTRATCOM), providing technical advice and assistance to geographic Unified Combatant Commands conducting consequence management operations in response to chemical, biological, radiological, nuclear and high-yield explosives incidents outside its AOR (i.e., continental United States (CONUS), Alaska, Canada, Mexico, and the US approaches). Within the United States, for accidents involving oil spills or other release of chemical, biological, radiological or explosive threats, the coordinating center is the National Response Center (NRC) of the Department of Homeland Security.
The commander also wears a multinational "second hat" as the Commander, US Element, North American Air Defense Command (NORAD) and will normally be designated Commander, NORAD, a binational command of the US and Canada, responsible for aerospace warning and aerospace control for Canada, Alaska and the CONUS.
USNORTHCOM’s civil support mission includes domestic disaster relief operations that occur during fires, hurricanes, floods and earthquakes. Support also includes counter-drug operations and managing the consequences of a terrorist event employing a weapon of mass destruction. The command provides assistance to a Lead Agency when tasked by DoD.
In providing civil support, USNORTHCOM generally operates through established Joint Task Forces subordinate to the command. An emergency must exceed the capabilities of local, state and federal agencies before USNORTHCOM becomes involved. In most cases, support will be limited, localized and specific. When the scope of the disaster is reduced to the point that the Lead Agency can again assume full control and management without military assistance, USNORTHCOM will exit, leaving the on-scene experts to finish the job.
Through NORAD the commander answers to both the US and Canadian governments. This responsibility includes planning for the binational Canada-US land and maritime defense of the Canada-US region, under the policies of the Canadian Department of National Defence and the U.S. Department of Defense.
The core units assigned to NORTHCOM may be augmented as required by the situation. Like all Unified Combatant Commands, it is intended to be a flexible headquarters to support joint and multinational operations.
- Joint Forces Headquarters-National Capital Region
- Joint Task Force-Alaska
- Joint Task Force-Civil Support
- Joint Task Force-North
- Standing Joint Force Headquarters-North
- Army North
- Air Force North
- U.S Fleet Forces Command
Joint Task Force North
JTF North supports federal law enforcement agencies in the identification and interdiction of suspected transnational threats within and along the approaches to the continental United States.
This is the Army Component Command of (USNORTHCOM). Prior to the establishment of NORTHCOM, this was the responsibility of the Fifth United States Army. Training and readiness previously under Fifth Army is now under First United States Army.
Air Force North
The Air Component of USNORTHCOM is First Air Force, whose headquarters are at Tyndall Air Force Base in Florida.
- U.S. Northern Command, About Northcom
- Joint Chiefs of Staff (14 May 2007), Doctrine for the Armed Forces of the United States p. III-10
- The U.S. Coast Guard, which is not part of the Department of Defense in peacetime, specifically has law enforcement authority. The Posse Comitatus Act clearly applies to the Army and Air Force, but its authority over the Navy is less clear.