Imagine if Hitler had listened to Italian foreign minister Ciano's advice at Salzburg in mid-August of 1939 and backed off his invasion plans of Poland.
Adolf Hitler listening to Ciano's advice regarding Poland? You're hilarious!
Anyway, I believe if there was no war ravaging the European continent at the time of the 1940 Presidential Election that the Republicans would win back the White House, with either Senator Arthur Vandenberg of Michigan or Senator Robert A. Taft of Ohio at the top of the Republican ticket, most likely the former than the latter. As for the Democrats, I would expect Postmaster General James A. Farley, with the backing of President Roosevelt to secure the nomination. Although Vice President Garner would have tried, his campaign would have been destroyed due to the "New Deal" coalitions control of the party machinery.