In The Balance by Harry Turtledove

It is May 1942. War is raging around the world. Major Heinrich Jager is fighting for the Third Reich on the Eastern Front. George Bagnall is a flight engineer for the RAF. Ludmila Gorbunova is a pilot, but for the Soviet Air Force. Moishe Russie is a Jew in the Warsaw Ghetto. Liu Han is a Chinese peasant woman. Jens Larssen is an American physicist on the Manhattan Project. Sam Yeager us a minor league outfielder and science fiction fan from Nebraska. Atvar is the fleetlord of the invading spaceships of the Race.