U.S. forces have already begun withdrawing from areas along Syria's border with Turkey as Turkish troops prepare to invade, U.S.-backed Kurdish forces, the Britain-based Syrian Observatory for Human Rights, and the Kurdish Hawar news agency reported Monday. On Sunday night, the White House said that President Trump had given his assent to the incursion in a phone call with Turkish President Recep Tayyip Erdogan. The U.S. "will not support or be involved in" Turkey's operation and "will no longer be in the immediate area," the White House said.

Trump's decision to withdraw the small contingent of U.S. troops for the area opens the way for Erdogan to attack the Kurdish-led Syrian Democratic Forces, America's most effective ally in routing the Islamic State in Syria. Erdogan considers the Kurdish fighters "terrorists." As recently as late September, senior U.S. officials said there was a consensus across the U.S. government to ensure the welfare of the Kurdish allies, including by protecting them from Turkey's persistent threats to attack them, The New York Times reports. Turkish officials privately said they thought Trump could be persuaded to withdraw U.S. troops.

U.S. officials told the Times late Sunday that the situation is fluid, U.S. forces are withdrawing to "get out of the way," and the U.S. military would stay neutral if Turkey and the SDF clash. The SDF is holding some 10,000 ISIS fighters and tens of thousands of other prisoners, and it's unclear what will happen to the militants if Turkey invades. ISIS has been gaining strength in Iraq and Syria and military officials in the region say ISIS isn't going anywhere soon.

"Allowing Turkey to move into northern Syria is one of the most destabilizing moves we can do in the Middle East," Rep. Ruben Gallego (D-Ariz.), a former Marine and Iraq War vet, tweeted. "The Kurds will never trust America again. They will look for new alliances or independence to protect themselves." Peter Weber