5.2 Harry Turtledove's World War

This is a series of four alternative history books in which aliens invade the earth in the middle of World War II. To survive and beat them, the allies and axis must join forces. Interesting.

I have now read the first three, Tilting the Balance, In the Balance and Upsetting the Balance

Yep, now I have read the fourth and final one in this series, Striking the Balance