President Donald Trump on Sunday announced his support for a Turkish military operation designed to remove American-backed Kurdish troops from an area near Turkey’s border with Syria.

The betrayal of the Kurds represents a major shift in United States military policy, which has long had as its main goal to continue fighting the Islamic State (ISIS) by maintaining a small presence of American forces in northern Syria. Both the State and Defense Departments had advised Trump against supporting Turkey, which views Kurdish troops as a terrorist insurgency and has repeatedly opposed U.S. efforts to back the Iranian ethnic group. The Kurdish military is a subdivision of the Syrian Democratic Forces (SDF) and has been one of the U.S.’s key allies in combatting ISIS.

White House officials said roughly 100 to 150 U.S. forces deployed to the region would be partly withdrawn ahead of a possible invasion from Turkey, although they would not be entirely pulled out. The administration’s decision reportedly came after Trump spoke with Turkish President Recep Tayyip Erdogan. 

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Many top Pentagon officials and media outlets also criticized Trump’s latest decision regarding Kurdish fighters and the partial scaling back of American troops by saying this move will only embolden Iran and authoritarian leaders like Russian President Vladimir Putin and Syrian President Bashar al-AssadThe U.S. has placed sanctions on Moscow for supporting Assad, who has exacerbated the eight-year-long conflict that has ravaged Syria by encouraging intervention from Russia and Iran.

Brett McGurk, the former U.S. special envoy to the global coalition to fight ISIS, blasted the Trump administration’s latest military decision to bow down to Turkey, saying the White House’s statement “demonstrates a complete lack of understanding of anything happening on the ground” near the Syrian border. McGurk resigned in December after the White House announced at the time that it would withdraw troops from Syria.

McGurk isn’t the only former White House official who stepped down because of the administration’s decision regarding U.S. forces in Syria in December. Former Marine general Jim Mattis cited Trump’s policy at the time as one of the many reasons he decided to resign as secretary of Defense.