Mr. Trump is also willfully ignorant about the rest of the world, about our military and its capabilities, and about government itself. He disdains expertise and experience while touting his own—such as his claim that he knows more about ISIS than America’s generals. He has no clue about the difference between negotiating a business deal and negotiating with sovereign nations.

All of the presidents I served were strong personalities with strongly held views about the world. But each surrounded himself with independent-minded, knowledgeable and experienced advisers who would tell the president what he needed to hear, not what he wanted to hear. Sometimes presidents would take their advice, sometimes not. But they always listened.

The world we confront is too perilous and too complex to have as president a man who believes he, and he alone, has all the answers and has no need to listen to anyone. In domestic affairs, there are many checks on what a president can do; in national security there are few constraints. A thin-skinned, temperamental, shoot-from-the-hip and lip, uninformed commander-in-chief is too great a risk for America.