If a U.S. president decides to launch a nuclear attack, the order is conveyed to a duty officer at the National Military Command Center at the Pentagon. The one-star commander executes the order in roughly a minute.
In the most urgent case, a land-based intercontinental ballistic missile will fire in two minutes. A submarine-launched missile will fire in 15 minutes.
That may be all it takes to end the world as we know it. The president has the sole authority to authorize the use of nuclear weapons, and once the order is given and acknowledged, there is no way to reverse it.