Trump declared U.S. troops would step aside for an expected Turkish attack on the Kurds, who have fought alongside Americans for years, but he then threatened to destroy the Turks' economy if they went too far.

Even Trump's staunchest Republican congressional allies expressed outrage at the prospect of abandoning Syrian Kurds who had fought the Islamic State group with American arms and advice. It was the latest example of Trump's approach to foreign policy that critics condemn as impulsive, that he sometimes reverses and that frequently is untethered to the advice of his national security aides.

"A catastrophic mistake," said Rep. Liz Cheney of Wyoming, the No. 3 House Republican leader. "Shot in the arm to the bad guys," said Sen. Lindsey Graham of South Carolina.

Trump said he understood criticism from fellow GOP leaders but disagreed. He said he could also name supporters, but he didn't.

Pentagon and State Department officials held out the possibility of persuading Turkey to abandon its expected invasion. U.S. officials said they had seen no indication that Turkey had begun a military operation by late Monday.

Trump, in late afternoon remarks to reporters, appeared largely unconcerned at the prospect of Turkish forces attacking the Kurds, who include a faction he described as "natural enemies" of the Turks.

"But I have told Turkey that if they do anything outside of what we would think is humane ... they could suffer the wrath of an extremely decimated economy," Trump said.

In recent weeks, the U.S. and Turkey had reached an apparent accommodation of Turkish concerns about the presence of Kurdish fighters, seen in Turkey as a threat. American and Turkish soldiers had been conducting joint patrols in a zone along the border. As part of that work, barriers designed to protect the Kurds were dismantled amid assurances that Turkey would not invade.

Graham said Turkey's NATO membership should be suspended if it attacks into northeastern Turkey, potentially annihilating Kurdish fighters who acted as a U.S. proxy army in a five-year fight to eliminate the Islamic State's so-called caliphate. Graham, who had talked Trump out of a withdrawal from Syria last December, said letting Turkey invade would be a mistake of historic proportion and would "lead to ISIS reemergence ."

This all comes at a pivotal moment of Trump's presidency. House Democrats are marching forward with their impeachment inquiry into whether he compromised national security or abused his office by seeking negative information on former Vice President Joe Biden, a political rival, from Ukraine and other foreign countries.

As he faces the impeachment inquiry, Trump has appeared more focused on making good on his political pledges, even at the risk of sending a troubling signal to American allies abroad.

"I campaigned on the fact that I was going to bring our soldiers home and bring them home as rapidly as possible," he said.

The strong pushback on Capitol Hill prompted Trump to recast as well as restate his decision, but with renewed bombast and self-flattery.

He promised to destroy the Turkish economy "if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits."

Sunday night the White House had said the U.S. would get its troops out of the way of the Turkish forces. That announcement came after Trump spoke by phone with Turkish President Recep Tayyip Erdogan.

One official described that White House announcement as a botched effort appeared aimed at making Trump look bold for ending a war. The official said attempts by the Pentagon and State Department to make the statement stronger in its opposition to Turkey's military action were unsuccessful.

That official, like others interviewed, was not authorized to speak on the record and was granted anonymity to comment.

The official added that Erdogan appeared to be reconsidering his earlier resolve because he was relatively quiet Monday. But damage done to relations with the Kurds could be irreparable.

An official familiar with the Erdogan call said the Turkish president was "ranting" at Trump, saying the safe zone was not working and that Turkey couldn't trust the U.S. military to do what was needed. And in reaction, Trump said the U.S. wanted no part of an invasion and would withdraw troops.

The announcement threw the military situation in Syria into fresh chaos and injected deeper uncertainty into U.S. relations with European allies. A French official, speaking on condition of anonymity on a sensitive topic, said France wasn't informed ahead of time. A Foreign Ministry statement warned Turkey to avoid any action that would harm the international coalition against the Islamic State and noted the Kurds had been essential allies. It entirely omitted any mention of the United States.

U.S. involvement in Syria has been fraught with peril since it started in 2014 with the insertion of small numbers of special operations forces to recruit, train, arm and advise local fighters to combat the Islamic State. Trump entered the White House in 2017 intent on getting out of Syria, and even before the counter-IS military campaign reclaimed the last militant strongholds early this year, he declared victory and said troops would leave.

Trump defended his latest decision, acknowledging in tweets that "the Kurds fought with us" but adding that they "were paid massive amounts of money and equipment to do so."

"I held off this fight for almost 3 years, but it is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home," he wrote.

In his later remarks, Trump asserted that American troops in Syria are not performing useful work. They are, he said, "not fighting." They are "just there," he said.

Among the first to move were about 30 U.S. troops from two outposts who would be in the immediate area of a Turkish invasion. It's unclear whether others among the roughly 1,000 U.S. forces in northeastern Syria would be moved, but officials said there was no plan for any to leave Syria entirely.

Bulent Aliriza, director of the Turkey Project at the Center for Strategic and International Studies, said that a U.S. withdrawal from Syria would be a major boost to Russia's position there.

He added that other allies in the region, including the Kurds, will "look at this withdrawal as U.S. unwillingness to stand up for its rights and maintain its alliances in the region."

House Minority Leader Kevin McCarthy, R-Calif., another strong Trump supporter, said in an appearance on "Fox & Friends" that he had concerns.

"I want to make sure we keep our word for those who fight with us and help us," he said, adding that, "If you make a commitment and somebody is fighting with you, America should keep their word."

Former Trump administration officials also expressed concern.

Nikki Haley, who served as U.S. ambassador to the United Nations, said the U.S. "must always have the backs of our allies, if we expect them to have our back. ... Leaving them to die is a big mistake."

Turkey considers the People's Protection Units, or YPG, an extension of the Kurdistan Workers' Party, or PKK, which has waged an insurgency against Turkey for 35 years.