National Command Authority
In the national security policy of the United States, the National Command Authority (NCA) is the combination of the President and Secretary of Defense, or their lawful successors, who can order the commitment of military forces. Military forces do retain, under locally defined rules of engagement (RoE), the authority to act in self-defense.
By the formal definition of command, as in command and control, the NCA is the method by which the top-level command decisions are conveyed to the operational forces.
The operational chain of command goes from the NCA to the Unified Combatant Commands:
- United States Central Command
- United States European Command
- United States Joint Forces Command
- United States Northern Command
- United States Pacific Command
- United States Southern Command
- United States Special Operations Command
- United States Strategic Command
- United States Transportation Command
While the NCA is considered the ultimate authority of a military under civilian control, the institution has special significance with respect to the use of nuclear weapons. One of the measures used to prevent accidental or rogue nuclear war is the "Two-Man Rule", in which all orders and actions relevant to the movement or use of nuclear weapons require the concurrence of two people, at any given level of command. Neither the President nor Secretary of Defense could order a nuclear strike on their own.
At the other extreme of the chain of command, there are two launch control officers in every intercontinental ballistic missile (ICBM) silo. Even when they have received and validated an order to fire, the process of arming and firing the missile requires agreement between the two officers. As a physical safeguard, the two officers must near-simultaneously turn two switches that are too far away from each other than one person can possibly reach. Each officer has an individual key for the locked switches, but even if one officer overpowered the other and gained control of both keys, it would be impossible to insert one key and turn the switch, and then rush to the other switch, insert the key, and turn it before the timer expired.
A series of successors to the President and Secretary of Defense exist, although once the statutory successors are eliminated, there are classified procedures by which there will always be a two-person NCA replacement. While no military personnel are in the usual succession, when the U.S. kept a strategic command post in the air at all times, originally on Air Force C-135 LOOKING GLASS aircraft and now on Navy E-6 TACAMO planes, it has been understood that if all higher authorities are incapacitated, the senior officer in the airborne command post, with concurrence from the second-ranking officer, can issue valid orders.
More authority is delegated to local military commanders who can authorize combat to protect their own troops, or with predelegated authority to act, for example, in defense of an ally.
In current usage, however, the NCA can order conventionally armed raids or other attacks, although the legal basis is complex and controversial. In principle, Congress has the authority to declare war, but the United States has not declared war since 1941. Other authorities have been used, basically at the discretion of the President.
There is Congressional understanding that prolonged combat would require Congressional authorization, certainly when the troops will be involved for longer than 60 days, and the War Powers Resolution would come into effect. In practice, Presidents have been able to get authorizations short of a declaration of war, such as the Gulf of Tonkin Resolution or the Authorization for the Use of Military Force after the 9-11 attacks.