Thankfully the people responsible for actually launching nukes tend to be pretty rational about it. Every time in history (other than the atomic bombs at Hiroshima and Nagasaki obviously) that people have been ordered to use them they have said no, across every country. There have been several close calls due to bad information, people thinking another country had shot them off etc.

I highly doubt that without another country initiating a strike the US military would ever shoot them off. The whole MAD concept is concretely understood by them, they wouldnt throw it out the window