National Command Authority (NCA) is a term used by the Department of Defense of the United States of America to refer to the ultimate source of lawful military orders. The NCA comprises the President of the United States (as commander-in-chief) and the Secretary of Defense jointly, or their duly deputized successors, i.e. the Vice President and the Deputy Secretary of Defense. The term also refers to communications with the commanding officers of the Unified Combatant Commands to put U.S. forces into action.
Authorization of a nuclear or strategic attack
Only the President can direct the use of nuclear weapons by U.S. armed forces, including the Single Integrated Operational Plan (SIOP). While the President does have unilateral authority as commander-in-chief to order that nuclear weapons be used for any reason at any time, the actual procedures and technical systems in place for authorizing the execution of a launch order requires a secondary confirmation under a two-man rule, as the President's order is subject to secondary confirmation by the Secretary of Defense. If the Secretary of Defense does not concur, then the President may in his sole discretion fire the Secretary. The Secretary of Defense has legal authority to approve the order, but cannot veto it.