A number of e-mails to the persuasive effect that Yalta could have turned out a lot differently if FDR and Churchill–but especially FDR–had stood up to the Soviets more vigorously during the preceding several years. Consider this, from MT:

An additional point: During the Warsaw Uprising (in which my mother fought) Churchill wanted to resupply the Polish Home Army with air drops of weapons and provisions. To stop this, Stalin refused to allow U.S. and British planes to land on Soviet soil — they could not make it to Warsaw and back without landing in Soviet controlled territory. Churchill told Roosevelt we should send them anyway — that Stalin would never actually prevent them from landing once they were in the air. Roosevelt refused. Stalin’s troops waited on the other side of the Vistula for the Germans to wipe out the Polish resistance (and thus the leaders of a free and independent Poland). If we had resuplied them, it would have sent a clear signal to Stalin that we would not acquiesce to his subjugation of Eastern Europe — and Yalta would have been a very different meeting….