The Soviets were complete jackasses to their allies. For example, they refused to allow American bombers to launch raids on Japan from eastern Russia, forcing the Allies to do a brutal island-hopping campaign to get them within range.

Then, when a few B-29s were too damaged to make the long flight back to the island and diverted to Russia, they confiscated the planes and imprisoned their crews until the end of the war.

Then, after the war, they dismantled two of the planes so they could figure out how to build a good airplane and claimed it as their own and called it the Tu-4.

Oh, and they also threw Andrei Tupolev into a gulag because he couldn't meet Stalin's insane demands.