BEIRUT, Lebanon — Turkey launched a ground and air assault on Wednesday against a Syrian militia that has been a crucial American ally in the fight against ISIS, days after President Trump agreed to let the operation proceed.

As Turkish warplanes bombed Syrian towns and troops crossed the border, the chaos in Washington continued, with President Trump issuing seemingly contradictory policy statements in the face of strident opposition from his Republican allies in Congress.

Mr. Trump acquiesced to the Turkish operation in a call with Turkey’s president on Sunday, agreeing to move American troops out of Turkey’s way despite opposition from his own State Department and military.

On Wednesday, hours after the operation began, he condemned it, calling it “a bad idea.”

By that time, Turkish fighter jets were streaking through the sky over Syrian towns, while artillery shells boomed overhead. Traffic was jammed with terrified civilians fleeing south in trucks piled high with possessions and children.