1. Turkey sent warplanes and troops into northeastern Syria aimed at flushing out a Kurdish militia, days after the U.S. withdrew forces near the border. Civilians fled.

Turkey had long planned to root out the militia, the Syrian Democratic Forces, which it considers to be a terrorist organization, but has been a crucial U.S. ally in the fight against the Islamic State. President Trump gave Turkey a green light on Sunday, sparking fierce debates in the U.S.

On Wednesday, Mr. Trump called the Turkish operation “a bad idea” but reiterated his opposition to “endless, senseless wars,” even as leading Republicans expressed outrage. Here are the basics of the conflict.