Kelly/Mattis can't or won't prevent Trump from using nukes.

[bpj62]

18. The Football

The bag which got its name during the Kennedy Administration contains a series of nuclear scenarios that the President can choose from. When the President reads the codes on the biscuit to the officer on the opposite end of the secure line and the code is authenticated the President can then decide which nuclear option he chooses to go with,. It could be land based ICBM, it could be bombers in the air force or from carriers, or submarines. Once the plan is entered into the computer the plan is authenticated via codewords by the Navy or Airforce and once authenticated the target is confirmed and the delivery platform locks the location in via satellite and the weapons are released for use. The President can stand down the launch prior to launch but once the weapon is in the air I am not certain if they can be self destructed. My apologies for the length but it really happens in minutes.

As for Mattis and Kelly restraining the President well that is a fantasy as both men would either be restrained or shot by the Secret Service whose sole mission is to ensure that the President is able to safely discharge his duties. Pretty bleak scenario.