The T-80 was the main battle tank used by the Soviet Union, it was used throughout the world.
T-80 tanks can be found during Terror Missions and in military bases throughout the world. They will typically be seen alongside Soviet Soldiers at pre built blockades and checkpoints trying to defend against the alien terror forces.
The tanks however are inoperable, and see no actual usage against the aliens. Considering how easily the tanks can be destroyed from plasma fire it can be speculated that the tank crews abandoned their position in the face of the alien attackers.