MOSCOW — 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 goal of ousting President Bashar 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 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,” said Frants Klintsevich, deputy head of the defense committee in the upper house of the Russian parliament. Driving the last rebel groups out of Aleppo within a few months is now “quite realistic,” he said.

It’s been almost a year since Putin stunned the U.S. and its allies by entering the conflict to battle Islamist militants and prop up an old ally, turning the tables on Western and regional powers intent on regime change. What started as a crackdown on peaceful protesters developed into a multifaceted proxy war that triggered Europe’s worst migrant crisis since World War II and facilitated the rise of Islamic State.

Opposition militias in Aleppo over the weekend managed to open a route out of besieged eastern neighborhoods where about a quarter of a million people live, but renewed strikes by government and Russian forces are preventing them from securing it, according to the Britain-based Syrian Observatory for Human Rights, which monitors the war through activists on the ground.

The joint siege of Aleppo got an unexpected boost last month when a failed coup in neighboring Turkey accelerated a rapprochement between President Recep Tayyip Erdogan and Putin, who called Turkey’s downing of a Russian fighter near the Syrian border last year a “stab in the back.”

With authorities in Ankara accusing the U.S. of complicity in the attempted coup and U.S. and European Union officials condemning Erdogan’s resultant purge, the Turkish leader is turning to Putin to forge a new strategic partnership. The two leaders will meet in St. Petersburg Tuesday, a day after Putin holds talks with the president of Iran, Assad’s other major benefactor.

Key to the Turkish detente for Putin is getting Erdogan to curtail the flow of arms and men to militias fighting his sworn enemy Assad, which he’s been doing since the failed coup, according to director Rami Abdurrahman of the observatory. Turkey has slowed weapons shipments from Arab countries for the Aleppo battle, while cutting its own flows significantly, he said.

“The Syrian regime wouldn’t have been able to besiege Aleppo had it not been for the Turkish-Russian rapprochement,” Abdurrahman said. “The military support is not what it used to be.”

The capture of Aleppo, whose eastern neighborhoods have been held by rebels since 2012, would give Assad control of more than 40 percent of Syrian territory and 60 percent of the population, leaving Islamic State with about 35 percent of the country, mostly desert, according to observatory estimates. The Kurds have about 15 percent of Syrian land, with the rest split between other groups.

Robert Ford, a former U.S. ambassador to Syria, said it’s now clear that Assad won’t be forced out, leaving the next president with few good options for dealing with a Kremlin-backed leader at the epicenter of Islamic extremism.

“Short of some kind of huge rethink in the U.S. and a whole set of relationships between the U.S. and partners in the region like Turkey and Saudi Arabia, Assad’s survival is no longer in question,” said Ford, who is now a senior fellow at the Middle East Institute in Washington. “The next administration will likely be left confronting a situation where a weakened but still powerful Syrian government under Assad controls the former population centers.”

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(Abu-Nasr reported from Beirut.)

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