...Is there any check on a president’s power to launch nuclear arms that could destroy entire cities or nations?
The short answer is no, though history suggests that in practice, there may be ways to slow down or even derail the decision-making process. No one disputes, however, that the president has an awesome authority...
...Then, in 1974, in the last days of the Watergate scandal, Mr. Nixon was drinking heavily and his aides saw what they feared was a growing emotional instability. His new secretary of defense, James R. Schlesinger, himself a hawkish Cold Warrior, instructed the military to divert any emergency orders — especially one involving nuclear weapons — to him or the secretary of state, Henry A. Kissinger.
It was a completely extralegal order, perhaps mutinous. But no one questioned it.
“Although Schlesinger’s order raised questions about who was actually in command,” Eric Schlosser writes in “Command and Control,” a 2013 book, “it seemed like a good idea at the time.”
Experts agree that the real nightmare of nuclear command centers not on launching attacks, but responding to them...
https://mobile.nytimes.com/2016/08/05/science/donald-trump-nuclear-codes.html