An earlier version of this article was published on Dec. 21, after President Trump said he wanted to withdraw American troops from Syria.

President Trump’s abrupt announcement last week that the United States would withdraw its troops from northeastern Syria has already begun to shift the power dynamic in a country that has endured years of war involving a tangled set of international actors.

The announcement preceded a Turkish incursion that turned what had been a largely peaceful Kurdish-administered stretch of northeastern Syria into a new flash point. Analysts and some of Mr. Trump’s own advisers had warned of this exact outcome for months, after he first teased the idea of a withdrawal.

Here are some of the parties that have the most to gain or lose.

The Winners:

President Bashar al-Assad of Syria, along with Russia and Iran

President Bashar al-Assad and his chief international backers, Russia and Iran, all stand to benefit from the troop withdrawal, which clears the way for Mr. Assad to tighten his once-tenuous grip on his battered country.