From the Document: "The U.S. President has sole authority to authorize the use of U.S. nuclear weapons. This authority is inherent in his constitutional role as Commander in Chief. The President can seek counsel from his military advisors; those advisors are then required to transmit and implement the orders authorizing nuclear use. But, as General John Hyten, the Commander of U.S. Strategic Command (STRATCOM), noted during his September 2016 confirmation hearing, his job is to give advice, while the authority to order a launch lies with the president. The President does not need the concurrence of either his military advisors or the U.S. Congress to order the launch of nuclear weapons. In addition, neither the military nor Congress can overrule these orders. As former STRATCOM Commander General Robert Kehler noted in a recent article, members of the military are bound by the Uniform Code of Military Justice 'to follow orders provided they are legal and have come from competent authority.' The President could delegate the authority to authorize the use of nuclear weapons to others in the chain-of-command (an option considered necessary during the Cold War), but they could not overrule the President."
CRS In Focus, IF10521
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html