National Guard units can be mobilized for federal active duty to supplement regular armed forces during times of war or national emergency declared by Congress,[27] the President[27] or the Secretary of Defense.[28] They can also be activated for service in their respective states upon declaration of a state of emergency by the governor of the state or territory where they serve, or in the case of Washington, D.C., by the Commanding General. Unlike U.S. Army Reserve members, National Guard members cannot be mobilized individually, except through voluntary transfers and Temporary Duty Assignments (TDY).