Turkey launched airstrikes and fired artillery across its border into northeastern Syria on Wednesday to open a military operation aimed at flushing out an American-backed militia, Turkish and Syrian officials said.

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 appeared to 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.

As voters in Iran danced in the streets, celebrating the landslide re-election  of a moderate as president, President Trump stood in front of a gathering of leaders from across the Muslim world and called on them to isolate a nation he said had “fueled the fires of sectarian conflict and terror.” That nation was Iran.