Upsetting the Balance (Worldwar, Book Three)

Alternate History is historical fiction that examines the ramifications of a change or changes in known history. Alternate History can use science-fiction devices -- such as time-travel -- to change history, or it can simply pick a point where, had things merely happened differently (for example: Abraham Lincoln decided not to go to the theater that night, or Henry VIII stayed married to his first wife), history might have taken another path altogether.
-- Alternate History with Science Fiction.
-- The series takes place during WWII and the warring Allied and Axis powers are forced to combine resources in order to combat an invading alien species that threatens to conquer the entire planet.