The National Guard is one of the United States' most significant military reserve elements. It is called upon in times of hostility to supplement regular forces in multiple arms of service. In addition to acting in this capacity, National Guard units also fulfill state functions when necessary. Thus, National Guard units are considered to shoulder both federal and state missions.
During times of war or national emergency as declared by Congress, the National Guard can be mustered for deployment in overseas hot zones, where they fall under the immediate authority of the Combatant Commander, the chief officer in that theatre of action. Ultimately, however, in times of war, the Guard is answerable to the president as commander-in-chief, just as are all other military units. However, unlike members of the Army Reserve, guardsmen and women can not be called up individually, but only through willful transfers and temporary assignment.
In addition to being "federalized" during wartime, the Guard also considers it part of its federal mission to maintain equipment, training and readiness levels during peacetime, so as to be ready if a situation necessitates. The Guard's state or domestic mission consists of service to individual or collective states, particularly when governors declare states of emergency. In these cases, the Guard may assist with evacuation orders, engineering efforts, cleanup or peacekeeping. As of 2014, National Guard units exist in all 50 states and the territories of Guam, Puerto Rico and the Virgin Islands, as well as in the District of Columbia.