Turtledove's Worldwar series

Turtledove's Worldwar series imagines an alternate history where an alien race, the Race, arrives during World War II. Initially mistaken for potential allies, the aliens intend to conquer Earth to access its resources. The series explores how humans, divided by their war, unite against the common extraterrestrial threat. It combines historical events with science fiction, showing military strategies, political struggles, and societal changes as humans and aliens clash in a desperate fight for survival during the 1940s.