"Only the president can order a nuclear launch," they said in the letter. "The pressures the system places on that one person are staggering and require enormous composure, judgment, restraint and diplomatic skill. Donald Trump does not have these leadership qualities. On the contrary, he has shown himself time and again to be easily baited and quick to lash out, dismissive of expert consultation and ill-informed of even basic military and international affairs – including, most especially, nuclear weapons."