BATTLEGROUND :During World War Two, in the merciless and freezing wilderness of Siberia, stranded German and American soldiers put their differences aside to exterminate hostile, self-replicating, alien parasites which threaten not only America and Germany — but all mankind.

I don't understand why it wouldn't be a German and Russian soldier since they are in Siberia.
Sort of, Enemy at the Gates but with Aliens

Hi pickerman 123,

How about

Stranded in the glacial wilderness of Siberia, WWII rivals, American and German soldiers must join forces and fight off an other-worldly / extraterrestrial / alien / paranormal enemy.

You can also ad:

that is now a threat to both armies / parties / sides / .....

I hope this helps