take look at this article

Presumably, by the time the POTUS got into that position, they gained the scruples not to launch a needless nuclear annihilation of another nation. That said, there's a chain of command that involves the Secretary of Defense who could refuse to relay the order (his codes —or those of the Asst. Sec. Defense in his absence— are required to launch) and then quickly call the Cabinet and Congress to report that the POTUS had gone crazy. The Cabinet can declare the President unfit in a letter to Congress.

Furthermore, there's a "must notify" part of the nuclear protocol that requires the DoD and whomever else inside the White House to notify senior leaders of the US government in the event of a nuclear launch. These leaders would include (and presumably not be limited to), Speaker of the House, President Pro Temp of the Senate, Majority/Minority Leaders of both houses of Congress, the Chairperson of the various oversight committees that are tied to war (Appropriations, Defense, Intelligence, Energy). The chain of command, that requires the Secretary of Defense and others to relay such an order is robust enough to handle such a statistically-impossible scenario of a "rogue president" ordering a launch.


.