Turkey launched a major military attack Wednesday on northern Syria.

Turkish ground forces crossed the border after airstrikes and artillery barrages, reports the Associated Press.

The Turks are determined to clear out Kurdish militia fighters who — until this week — had full backing from the U.S. in the fight against the Islamic State (ISIS).

President Trump apparently ended that alliance by giving Turkish President Recep Tayyip Erdogan a go-ahead for the attack on Sunday. Turkey considers the Kurds terrorists.

The White House said the relatively small number of U.S. troops in Syria’s northeastern corner (roughly 1,000) would move out the way and would not support or be involved in the operation.

Sources told Reuters that the Syrian Democratic Forces (SDF), which include the Kurdish militia, have halted operations against Islamic State forces still hanging on in parts of Syria. The SDF apparently continues to guard prisons holding around 11,000 ISIS fighters.

But in a statement, the SDF said the area was “on the edge of possible humanitarian catastrophe” and warned that the attack “will spill the blood of thousands of innocent civilians because our border areas are overcrowded.”

Panicked civilians are said to be fleeing the border region, and at least two civilians are reported to have been killed and others wounded Wednesday by a Turkish airstrike.

Two unnamed American military officials told the New York Times that the Trump administration “has cut off all support” for the Kurdish militia, apparently on direct orders from the president.

NATO’s secretary general, Jens Stoltenberg, urged Turkey, a NATO member, “to act with restraint” and to ensure that “the gains we have made in the fight against ISIS are not jeopardized,” the Times says.

A French spokesman says his government and those of Britain and Germany are drafting a joint statement that will be “extremely clear about the fact that we very strongly, very firmly condemn” the Turkish offensive.

Despite his seeming support for Turkey against America’s erstwhile allies, Trump issued a statement saying the U.S. “does not endorse this attack and has made it clear to Turkey that this operation is a bad idea.”  

“Turkey,” the statement says, “has committed to protecting civilians, protecting religious minorities, including Christians, and ensuring no humanitarian crisis takes place — and we will hold them to this commitment.”

But a senior adviser to the Turkish president told CNN that Erdogan Donald Trump “reached an understanding over precisely what this operation is.”

His words aside, Trump’s actions have alarmed both Republicans and Democrats in Congress, turning even staunch political allies against him.

One of those allies, Sen. Lindsey Graham (R-SC), addressed Turkey via Twitter on Tuesday, warning the country not to go ahead with the operation.

“To the Turkish Government: You do NOT have a green light to enter into northern Syria,” Graham wrote. “There is massive bipartisan opposition in Congress, which you should see as a red line you should not cross.”

But the line has been crossed — and no one can be certain of the consequences.