Get Congress or the Cabinet to invoke the 25th amendment. Pass the pass the Restricting First Use of Nuclear Weapons Act of 2017. Impeach him. Pass a law that requires the approval of the Secy of Defense or the Secy of State for a nuclear first strike. https://www.commondreams.org/views/2017/10/16/5-myths-about-war-north-korea-make-americans-think-they-are-safe