If Trump decides to launch a preemptive nuclear strike against any target, then who has the authority to say 'no'?
The military are bound to obey lawful orders, and the law gives the POTUS the authority to give such an order.
The people who launch the strike are not expected to second guess the orders they are given; they are trained to verify that the order is genuine, and if so, to comply with it.
The idea that the people selected by the military to man the silos, subs and planes might not be explicitly chosen for their inclination to obey the order to use the weapons entrusted to them is very silly.
There is certainly no reason to be confident that there would be any effective push-back from the lower ranks against a direct order confirmed as coming from the Commander in Chief and holder of the Executive Power of the United States of America. Widespread mutiny on the basis of a single order to launch against a target which does not have the ability to destroy the USA in a retaliatory strike is unthinkable. (Particularly as the people carrying out the orders would have no way to know if they are making a first strike, or are retaliating against a target that just attacked the US or a US ally).
The orders arrive; you go through the drill exactly as you were trained to do it. There's neither the time, nor the access to external information, nor the inclination, to do anything else.
Militarily units tend not be be full of pacifists who respond to a lawful order to use extreme force by saying "I won't do that, sir. People could be
killed if I did that".