Some of the nearly 25,000 U.S.-backed forces made some of their biggest gains in Syria in recent months when they retook villages near the hotly contested city of Aleppo with the help of Russian airstrikes. The Russian attacks targeted other opposition groups, and the U.S.-backed coalition exploited them to gain back parts of the city.

The Russian strikes were not designed to help those U.S.-backed forces, nor were the groups coordinating with the Russians. But nevertheless, the air attacks allowed the U.S.-created Syrian Democratic Front to make advances, four defense officials told The Daily Beast.

“It’s not a planned thing. It is not something Russians are trying to do. It is happenstance,” one U.S. official told The Daily Beast. “The net sum of everything the Russians are doing helps Assad and prolongs the war.”

And now, U.S. officials fear such success could create a new alliance between its proxy forces in Syria and the Russian military, which American generals have repeatedly called a potential “existential” threat to the United States.