U.S. Northern Command
From The Official US Northern Command Web Site.

U.S. Northern Command (USNORTHCOM) was established Oct. 1, 2002 to provide command and control of Department of Defense (DOD) homeland defense efforts and to coordinate defense support of civil authorities. USNORTHCOM defends America's homeland — protecting our people, national power, and freedom of action.

USNORTHCOM’s specific mission:

USNORTHCOM anticipates and conducts Homeland Defense and Civil Support operations within the assigned area of responsibility to defend, protect, and secure the United States and its interests
USNORTHCOM’s AOR 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, the Straits of Florida, portions of the Caribbean region to include The Bahamas, Puerto Rico, and the U.S. Virgin Islands. The commander of USNORTHCOM is responsible for theater security cooperation with Canada, Mexico, and The Bahamas.

USNORTHCOM consolidates under a single unified command existing missions that were previously executed by other DOD organizations. This provides unity of command, which is critical to mission accomplishment.

USNORTHCOM plans, organizes and executes homeland defense and civil support missions, but has few permanently assigned forces. The command is assigned forces whenever necessary to execute missions, as ordered by the president or secretary of defense.

Civil service employees and uniformed members representing all service branches work at USNORTHCOM’s headquarters located at Peterson Air Force Base in Colorado Springs, Colo.

The commander of USNORTHCOM also commands the North American Aerospace Defense Command (NORAD), a bi-national command responsible for aerospace warning, aerospace control, and maritime warning for Canada, Alaska and the continental United States.

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 Primary Agency when tasked by DOD. Per the Posse Comitatus Act, military forces can provide civil support, but cannot become directly involved in law enforcement.

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 Primary Agency can again assume full control and management without military assistance, USNORTHCOM will exit, leaving the on-scene experts to finish the job.

http://www.northcom.mil/About/index.html