Now, advisers who had opposed the move must get on board with the president’s abrupt decision.

Trump had never hidden his distaste that U.S. troops were still stationed in Syria, about 2,000 of them concentrated in the country’s northeast. But alarm bells began to ring in earnest after Trump held a rally in Richfield, Ohio, on March 29. “We’ll be coming out of Syria, like, very soon,” he said. “Let the other people take care of it now.”

The president’s senior-most advisers sought clarity after the March remarks, according to two people familiar with events over the last several months who asked not to be identified discussing private deliberations: Was this just another stump speech aimed at firing up the base? How serious was he?

Since then, those advocating for the U.S. to stay have included National Security Adviser John Bolton, Secretary of State Michael Pompeo, and the two U.S. envoys tasked with Syria and the defeat of Islamic State, James Jeffrey and Brett McGurk. Defense Secretary Jim Mattis said publicly less than two weeks ago that “there’s more work to be done” in Syria.

After intense internal debate, Trump’s advisers believed they had managed to talk him out of an immediate withdrawal, according to the people. But while military planners at the Pentagon and U.S. Central Command had no orders to do so, officials took the March remarks as their cue and began developing plans to withdraw U.S. troops should the order come.

It came Wednesday in classic Trump fashion—Twitter posts in the morning and evening and a Facebook video recorded at the White House. “After historic victories against ISIS,” Trump tweeted in the evening, “it’s time to bring our great young people home!”

“Getting out of Syria was no surprise,” Trump said Thursday on Twitter (TWTR), adding that he’d “been campaigning on it for years.”

For a time, Trump’s advisers sought to convince the president that U.S. troops must remain to “ensure the enduring defeat of ISIS.” All those involved understood that phrase to mean U.S. and coalition troops would remain through any political transition that would install a Syrian government capable of controlling the whole country.

The other danger they cited is Iran. U.S. officials managed for a time to persuade the president—who has built his Middle East policy on a vow to counter Iran’s influence—that troops should stay until Iran withdrew its own advisers and forces from Syria.

With the withdrawal plans shelved until now, Pompeo repeatedly highlighted Iran’s “malign” activity in Syria and the need to counter it. Bolton said in September U.S. troops wouldn’t leave “as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias.”

On Dec. 11, McGurk, the administration’s special envoy to the U.S.-led coalition to defeat Islamic State, told reporters that there remained “a significant concentration of the most hardened ISIS fighters” in Syria.

“We can’t just pick up and leave,” McGurk said.

On Wednesday, Trump decided to do exactly that, although administration officials refused to predict the pace of withdrawal.

‘Huge Mistake’

Lawmakers from both parties blasted the decision, saying the U.S. departure also will leave Russia as a prime player shaping Syria’s future and that Turkish President Recep Tayyip Erdogan may deliver on his vow to attack Kurdish forces in Syria that have been loyal U.S. allies but that Erdogan considers terrorists.

“Trump’s Syria decision is a huge mistake,” said Max Hoffman, associate director of national security and international policy at the Center for American Progress. “Now he has thrown off the experts, and hundreds or thousands of people will die because of his pique and ill-discipline.”

White House spokeswoman Sarah Huckabee Sanders seemed to muddy the waters further with a statement Wednesday that the victories over ISIS “do not signal the end of the Global Coalition or its campaign.”

“The United States and our allies stand ready to re-engage,” Sanders said.

American officials insist that Trump’s decision doesn’t change the campaign to make sure Islamic State can’t regain a foothold in Iraq, Syria’s neighbor. But coalition partners were clearly rattled.

“Much remains to be done, and we must not lose sight of the threat” posed by Islamic State, a U.K. government official said. “As the United States has made clear, these developments in Syria do not signal the end of the Global Coalition or its campaign.”