If the president is not at the White House or other location with secure communication, he or she would use the so-called nuclear football to order the use of nuclear weapons. The football, or Presidential Emergency Satchel, is a briefcase containing various items, including a book laying out various attack options, from striking a small number of military targets to launching an all-out attack against Russian nuclear forces, military installations, leadership facilities, military industry, and economic centers. This briefcase is carried by an aide who stays near the president at all times.
The president carries a card—the “biscuit”—with a code that changes periodically and would be used to authenticate a launch order. To order the use of nuclear weapons, either first or in retaliation, the president would call the Pentagon's National Military Command Center—known as the War Room—read the code on the biscuit to confirm that he or she is indeed the president, and specify what attack option to use. (Our proposal to require approval of a nuclear attack order by two officials in the line of presidential succession, “How to limit presidential authority to order the use of nuclear weapons,” can be found here.)
After confirming the president’s identity, the Command Center would send an encrypted launch order to aircraft pilots, the underground crews that launch land-based missiles, and/or the submarine crews that launch submarine-based missiles.
For land-based missiles, it would be a matter of minutes from the presidential order to when missiles would leave their silos.
If the War Room is unable to function during a crisis, the War Room’s role is taken over by Strategic Command.