DOHUK, Iraq — Syrian government forces streamed into the country’s northeast on Monday, seizing towns where they had not set foot in years and filling a vacuum opened up by President Trump’s decision to abandon the United States’ Syrian Kurdish allies.

Less than a week after Turkey launched an incursion into northern Syria with Mr. Trump’s assent, President Bashar al-Assad of Syria, considered a war criminal by the United States, has benefited handsomely, striking a deal with the United States’ former allies to take the northern border and rapidly gaining territory without a fight.

In addition to Mr. al-Assad, Mr. Trump’s decision to pull American forces out of the way has also quickly redounded to the gain of Russia and Iran, as well as the Islamic State, as the retreat reconfigures battle lines and alliances in the eight-year war.

“For the Syrian regime and Russia, the Americans are leaving, so that is a big achievement,” said Hassan Hassan, a Syria analyst at the Center for Global Policy. “In just one day, gone. They don’t have to worry about what this presence means for the future.”