Vladimir Putin may be on the cusp of a pivotal victory in Syria’s civil war that would make it much harder for the U.S. to achieve its stated goal of ousting Bashar al-Assad without a major military escalation.

Assad’s troops, backed by Russian air power, are bearing down on rebels entrenched in Aleppo, Syria’s most populous city before fighting erupted in 2011. Reclaiming Syria’s commercial capital would give Assad control over all major population centers and cement his hold on a contiguous swath of land from Turkey to Jordan that makes up almost half of the country.

“Russia will stick to its guns in Syria and show the whole world we are right,” Frants Klintsevich, deputy head of the defense committee in the upper house of parliament, said by phone from Moscow. Driving the last rebel groups out of Aleppo within a few months is now “quite realistic,” he said.