A Century of Progress — Fred Saberhagen

For centuries two meg-powers from the future have been waging war for control of timelines of history. Alan Norlund, a WWII airman vet, is promised a lifesaving cure for his granddaughter, if he will undertake a mission 50 years back in time to the 1933 Chicago World''s Fair. Hitler with his ""angels"" from the opposing power would like to take over our timeline and 1933 Chicago may be just the place to start.