National Command Authority (NCA) is a term that was used by the Department of Defence of the United States of America to refer to the ultimate source of lawful military orders.
Refer to DEFCON.
The NCA was first alluded to in a 1960 Department of Defence document. It included at least the president of the United States as commander-in-chief and sometimes the vice president, secretary of defence, or the Joint Chiefs of Staff, and/or their alternatives and successors. The term has no statutory or constitutional basis and was replaced in 2002 in favour of explicitly referring to the president and/or the secretary of defence.
The term also refers to communications with the commanding officers of the Unified Combatant Commands to put US forces into action.
Authorisation of a Nuclear or Strategic Attack
Only the president can direct the use of nuclear weapons by US Armed Forces, through plans like OPLAN 8010-12 (military operation plan). The president has unilateral authority as commander-in-chief to order that nuclear weapons be used for any reason at any time.