Nuclear strikes and responses to attacks on the US are handled through a chain of command set up decades ago to ensure civilian control of military forces as mandated by our Constitution. Recently the Senate fussed over Trump's role. "We are concerned that the President of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic that he might order a nuclear weapons strike that is wildly out of step with US national security interests," said US Sen. Chris Murphy, D-Connecticut, inspiring articles like this one in The Atlantic.
Here’s what really happens. Our military sensors detect threat indications like a hostile missile launch. The Deputy Director of the National Military Command Center at the Pentagon sets up a teleconference. It’s like a conference call, except very secure. The teleconferences have different names like Significant Event Conference or Air Threat Conference, as we learned after September 11. The DDO is a one-star general or admiral who’s had additional training to lead his or her watch team. The NMCC establishes the link between the National Command Authority (President and Secretary of Defense) and the combatant command who will carry out the orders, if any. Other participants such as the Chairman, NORAD, White House situation room, and Air Force crisis action team may join the call. The DDO keeps all parties linked together.
If necessary, they’ll call in the President. The President’s military aide connects him to the secure teleconference. That’s why the Mil Aide carries “the football” a briefcase with information for connecting to the NMCC’s threat teleconference from any location including aboard Air Force One.
As you can see, President Trump is far from alone in this process. Secretary of Defense James Mattis would be on the line along with Chairman Dunford. Combatant commanders can talk with Trump about what they are seeing and recommend responses. It’s not inconceivable to have a dozen or more participants.
What happens next depends on the situation.
Air attack on the US or Canada? At NORAD, General Lori Robinson will scramble fighters sitting alert under NORAD control.
Inbound missile from North Korea? Gen Lori Robinson again, activating Fort Greely, Alaska’s interceptors.
Russian ICBMs barrage? Trump decides whether to order Gen John Hyten, STRATCOM, to execute the nuclear war plan using forces from the triad of B-2 and B-52 bombers. Ultimately it is the duty of military officers at the missile silo, on the submarine or airborne in a B-52 or B-2 to turn keys and ignite hydrocarbons to deliver a nuclear strike. Read retired STRATCOM Gen Kehler's Senate testimony here.
By the way, Congress set up this chain via legislation long ago. Be sure Trump has been drilled in this procedure – a key finding after 9/11 was to train incoming administration personnel on threat conference procedures.