BEIRUT, Lebanon — Feeling betrayed by the United States, its Kurdish allies in Syria asked the Syrian government on Friday to protect them from possible attack by Turkey.

The request surprised some American officials and could help open the way for the forces of President Bashar al-Assad of Syria, backed by Russia and Iran, to start retaking the Kurdish-held part of the country near Turkey’s border.

That would be a big step toward Mr. Assad’s goal of reclaiming all of Syria, upended by almost eight years of war.

It was also the first sign that President Trump’s abrupt announcement last week that he was withdrawing American troops from Syria was not only shifting alliances in the conflict but directly benefiting Mr. Assad — a brutal autocrat once described by Mr. Trump as an “animal” responsible for chemical attacks and other atrocities.