Well, FDR worked very hard to prepare us for war in spite of the opposition of the pacifists in his own party. He did all he could to keep the UK afloat through Lend Lease and other means in spite of opposition in Congress. He started the draft in spite of opposition of his Secretary of War. When the war started, he worked with capitalist and even brought them into his government through, for example, the “Dollar a Year” program. He was a good war leader. He chucked Henry Wallace and picked HST as vice president just in time.
FDR would’ve preferred Justice “Wild Bill” Douglas as his VP in 1944. Douglas scarcely differed from the pro-Soviet Wallace. Truman, of course, was no prize.