See the timeline.
What does it mean for the U.S. president to "push the button"?
Recently, retired General Michael Hayden was asked about remarks attributed to Donald Trump regarding U.S. nuclear weapons. One of the hosts of the show, Joe Scarborough, asked Hayden how quickly a U.S. President could launch nuclear weapons. Hayden replied "The system is designed for speed and decisiveness. It is not designed to debate the decision." What Hayden is describing is "Launch Under Attack" -- the option to launch U.S. nuclear-armed ICBMs after enemy missiles have been launched, but before they strike targets in the United States.
A new timeline, produced by the James Martin Center for Nonproliferation Studies for the NTI website, illustrates how much time is lost to determining that an attack is underway (or whether it is a false alarm) and the time needed to physically launch U.S. ICBMs, and how little is left over for a U.S. President to weigh options and consider alternatives.