The lessons that many in the Pentagon and the intelligence agencies learned in post-9/11 era – that deployed forces are key to stopping terrorists before they reach American shores and vital to maintaining the alliances that keep the world safe – never resonated with Trump. He is far more engaged with the idea of using the military to secure the Mexican border than to counter Russia, Iran, North Korea and China.

And now, by ordering the small American force of 2000 troops to leave Syria, Trump is about to turn his theory into practice. He is doing so to the quiet horror of many of his senior aides, who have long argued that to pull out of Syria (or Afghanistan, another conflict in which Trump has said the United States has no legitimate long-term role) is to ignore the lessons of the past two decades.

U.S. President Donald Trump overrode his top national security aides, blindsided U.S. ground commanders, and stunned lawmakers and allies with his order for U.S. troops to leave Syria, a decision that upends American policy in the Middle East.

The result, said current and former officials and people briefed on the decision, will empower Russia and Iran and leave unfinished the goal of erasing the risk that Islamic State, or ISIS, which has lost all but a sliver territory, could rebuild.

Trump was moving toward his dramatic decision in recent weeks even as top aides tried to talk him out of it, determined to fulfill a campaign promise of limiting U.S. involvement militarily abroad, two senior officials said.

The move, which carries echoes of Trump’s repudiation of the Iran nuclear deal and the Paris climate change accord, is in keeping with his America First philosophy and the pledge he made to end U.S. military involvement.

A former senior Trump administration official said the president’s decision basically was made two years ago, and that Trump finally stared down what he considered unpersuasive advice to stay in.

“The president won. His inclination was always not to be there,” said the former official who is close to the White House, saying a variety of senior advisers had all argued against pulling out.