In just a few weeks, the American withdrawal from northern Syria dramatically reordered power in the country after eight years of civil war.

Here's how Syria looked three weeks ago: American-backed Kurds controlled the northeast, while the Syrian government controlled most of the rest.

Turkish forces were already in northwest Syria, but Turkey had long wanted to establish a roughly 20-mile-deep buffer along the whole border. It sees the Kurdish-led forces there as terrorists.

But those same Kurdish-led forces were fighting alongside the United States against ISIS. Roughly 1,000 American troops were positioned across bases and outposts in northern Syria — including two bases near the Turkish border.

On Oct. 6, President Recep Tayyip Erdogan called President Trump and said Turkey would invade. Mr. Trump immediately ordered troops to withdraw from those two bases — allowing Turkey to move in and changing years of American policy.

Turkey followed by bombing Syrian towns and sending ground troops over the border. The offensive displaced around 200,000 people and killed more than 200 civilians.

Turkey reached the M4 highway, a crucial road that connects two Kurdish population centers — Kobani in the west and Qamishli in the east. Control the M4 and you cut the Kurdish-led forces in two.

Abandoned by the United States, the Kurds had few options. In a dramatic shift, they made a deal with an American enemy — the Syrian government. The partnership opened the door for President Bashar al-Assad to try to regain control of the whole country.

The deal also allowed Russia, which backs Mr. al-Assad, to help contain the Turkish incursion into Syria and enter areas that were under American protection just days before.

As Americans continued to evacuate their bases, the United States signed a cease-fire deal with Turkey that demanded Kurdish forces leave the area where Turkey was advancing. The deal essentially gave American assent for Turkey's plans in northern Syria.

Five days later, Russia and Turkey divided up northern Syria: Turkey got the area it had captured and a promise that the Kurds would push back 20 miles. Russia and Mr. al-Assad got the rest. It's unclear what will happen with the land the Kurds still control.

The changing landscape also threw a wrench into U.S. plans targeting the ISIS leader, Abu Bakr al-Baghdadi. The U.S. had to speed up the raid, and the Kurds, feeling betrayed, suspended their crucial security cooperation. Al-Baghdadi was killed despite the obstacles.

Four American adversaries gained under Syria's new geography. Mr. al-Assad's control expanded. Iran, a long-time Syrian ally, could gain a long supply route to its proxy Hezbollah in Lebanon.