But veteran diplomats, foreign policy experts and key lawmakers fear that Mr. Trump is squandering American power and influence in the world with little to show for it. By pulling troops out unilaterally, they argue, Mr. Trump has emboldened America’s enemies and distressed its allies. Friends like Israel, they note, worry about American staying power. Foes like North Korea and the Taliban learn that they can achieve their goals without having to pay a price.

“It’s hard for me to divine any real strategic logic to the president’s moves,” said John P. Hannah, a senior counselor at the Foundation for Defense of Democracies and a former national security adviser to Vice President Dick Cheney. “The only real connective tissue I see is the almost preternatural isolationist impulse that he invariably seems to revert to when left to his own devices internationally — even to the point that it overrides his supposed deal making instincts.”…

“When he canceled the South Korea military exercises, the only person he consulted was Kim Jong-un,” Mr. Brigety said. “The decision to abandon the Kurds came after a brief phone call with Erdogan. So they weren’t taken because he had personally reflected on the strategic disposition of American forces around the world. They were taken after he took the counsel of strongmen over that of his own advisers.”