Mr. Bunny,
I am suprised your possibility of a Dewey victory in 1944 has not resulted in more responses. Not only is it a real possibility but it could have far reaching effects.
FDR was very sick and weak by 1944 and only a maginificent act of will got him through the campaign. A serious, even if non-fatal illness (the flu?) in Septmeber or October of 1944 could have prevented him from campaigining and held him up to the public as weak and not up to another term. Alternatively, a military disaster at Normandy or in the Pacific in middle or late 1944 could have led to an erosion of his popularity. Finally, a major scandal, either financial (defense contracts?) or sexual (Sumner Welles?) could have turned just enough voters against FDR to give Dewey a victory.
A Dewey victory would put a young and relatively inexperienced President in office for the climatic battles at the end of the war and the important post war conferences. A few thoughts:
1. Who would be in Dewey's cabinet? He might retain Stimson at War and try to bring in a major Democrat for Navy. Perhaps Forrestal? I am not sure that John Foster Dulles was the major GOP spokesman for foreign policy that he became after the war so perhaps he is not a lock for State. Perhaps Wilkie if he lives long enough? Or perhaps Vandenburg? He might look to the young Governor of California, Earl Warren for Justice or Interior. His trusted aid and campaign manager, Herbert Brownell, would probably be in the White House or at Justice or the Post Office. Harold Stassen might become a respected cabinet secretary instead of a political joke.
2. The Bomb. Would Dewey react to the news about the existence of the Mnahattan Project in the same manner as Truman? Would he have the same type of "wise men" committee to discuss it's possible use and recommend alternatives as Truman relied on? I think yes, and I think he would use the bomb in approximately the same way Truman did. I think the reasons for using the bomb against Japan would impell any President to do what Truman did.
3. One interesting effect of a Dewey Presidency in 1945 would be on the future of the GOP. Dewey was quite liberal on civil rights (for his time) and he might take the step of integrating the armed forces and proposing civil rights legislation which Truman did in OTL. The Southern Democrats would oppose him and the GOP would continue to be the party of Lincoln. Dewey might also propose various social programs to deal with post war challenges such as housing and health (as he did as Governor of NY) which would keep the GOP a moderate party for longer than in OTL.
I welcome your comments and corrections.
AH