FDR was completely correct to mislead Churchill and work with Stalin. The allies needed Russia to beat the Nazis, just like the Soviets needed the British and Americans. Hitler was a far greater threat at the time. Where FDR may have messed up though was in sitting our troops on the banks of the Elbe for 3 weeks letting the Soviets take Berlin. Yes, it cost 250,000 Soviet troops, but it also led to the post-war partition of Germany.