One of the most important events for a new President is the "nuclear briefing." After his, Donald Trump told reporters that a "nuclear holocaust would be like no other."
At his briefing, the President learns that he has the sole authority to order the use of nuclear weapons and that no second-vote is required. He is told his order can be transmitted in minutes, with no possibility of countermanding it afterwards in the event of a false alarm. He will be told that this system, designed for speed and decisiveness, exists because the United States maintains a policy called "Launch Under Attack" -- the option to fire U.S. nuclear-armed ICBMs after Russian missiles have been launched, but before they destroy targets in the United States about thirty minutes later.
The following timeline illustrates how much of that thirty minutes is lost to determining that an attack is underway (including assessing whether it is a false alarm), while still leaving enough time to launch U.S. ICBMs. The U.S. President is left with at best 2-3 minutes to weigh options and consider alternatives.