Syrian troops have entered several northern towns and villages near Turkish border

AKCAKALE, Turkey — Syrian government troops moved into towns and villages in northeastern Syria on Monday, including the flash point region of Manbij, setting up a potential clash with Turkish-led forces advancing in the area as long-standing alliances in the region began to shift or crumble following the pullback of U.S. forces.

The Syrian military’s deployment near the Turkish border came after Syrian Kurdish forces previously allied with the U.S. said they had reached a deal with President Bashar Assad’s government to help them fend off Turkey’s invasion, now in its sixth day.

Assad’s return to the region his troops abandoned in 2012 at the height of the Syrian civil war is a turning point in Syria’s eight-year civil war, giving yet another major boost to his government and its Russian backers and is likely to endanger, if not altogether crush, the brief experiment in self-rule set up by Syria’s Kurds since the conflict began.

The rapidly changing situation was set in motion last week, when President Trump ordered American troops in northern Syria to step aside, clearing the way for an attack by Turkey, which regards the Kurdish fighters as terrorists. Since 2014, the Kurds have fought alongside the U.S. in defeating the Islamic State in Syria, and Trump’s move was decried at home and abroad as a betrayal of an ally.

Faced with unrelenting criticism, Trump said Monday he was putting new sanctions on Turkey, halting trade negotiations and raising steel tariffs in an effort to pressure Ankara to stop its offensive. He also said in a written statement that the roughly 1,000 U.S. troops he has ordered to leave Syria will “redeploy and remain in the region” to prevent a resurgence of the Islamic State threat. Trump also confirmed that the small number of U.S. troops at a base in southern Syria will...