Trump Unites the Left and Right, Against His Plans in Syria and Afghanistan

Those frustrations are widely held by NATO allies who have been America’s partners in Afghanistan.

“It’s been getting increasingly harder to explain to European publics why we need to stay there,” said Tomas Valasek, a former NATO ambassador from Slovakia who is the director of Carnegie Europe. “Perhaps Trump’s tendency to disrupt things, including by withdrawing from Afghanistan, may not be such a bad idea after all.”

In Afghanistan, the inevitability of an American withdrawal has been in the air since at least 2014, when Mr. Obama ordered a major troop reduction. Haroun Mir, an Afghan analyst, said it was understandable that Mr. Trump would be fed up, given the chaos on the ground and lack of progress in negotiating a political settlement.

Afghanistan’s leaders, mired in political infighting and corruption, need to see the partial pullout as a sign that they don’t have much time, he said.

But that is not without risk, Mr. Mir warned, because if the Taliban returned, it would haunt the Americans “that they were defeated by a ragtag force after 17 years of fighting them.”

However precipitously Mr. Trump acted, he was channeling the same reservations that Mr. Obama had. Both presidents questioned the open-ended nature of these campaigns, pressed their advisers to define success, and faced the problem of “mission creep.” Mr. Trump’s national security adviser, John R. Bolton, recently vowed that the United States would not leave Syria as long as Iran, or its proxies, were active there.

“Suggesting they stay until Iranian influence was gone was an unachievable goal and a recipe for potential escalation for a deployment that the Trump people have never been particularly transparent about,” said Benjamin J. Rhodes, a former deputy national security adviser to Mr. Obama.

Expanding the mission to resisting Iran, he pointed out, also “has no distinct congressional authorization.”