He sits in the White House at night, watching television or reading social media, and through Twitter issues instant judgments on what he sees. He channels fringe ideas and gives them as much weight as carefully researched reports. He denigrates the conclusions of intelligence professionals and then later denies having done so. He thrives on conflict and chaos.

For a capital that typically struggles to adjust to the ways of a new president every four or eight years, Mr. Trump has posed a singular challenge. Rarely if ever has a president been as reactive to random inputs as Mr. Trump. Career government officials and members of Congress alike are left to discern policy from random Twitter posts spurred by whatever happened to be on television when the president grabbed the remote control…

Yet some of his own advisers also privately worry about his penchant for picking unnecessary fights and drifting off message. They talk about taking away his telephone or canceling his Twitter account, only to be dismissed by a president intent on keeping his own outlets to the world.