It is hard to say what would have happened if the United States did not invade mainland Europe on June 6, 1944. Counterfactual history is always hard to guess—there might be some variables that could change the outcome of events. By June 1944, the Soviets had broken the sieges at Stalingrad and Leningrad and were beginning to roll the Nazis back across Eastern Europe. The Americans had landed in Italy and would soon liberate Rome in July 1944, but the Nazis maintained a furious defense in the Alps, making a southern invasion of Germany more difficult than anticipated. The British and Americans were bombing mainland Germany nearly 24 hours a day, but Hitler was no closer to being killed or overthrown than he was in 1939. If D-Day did not happen, the Soviet Union may have taken all of Germany and Austria with the Soviet army, killing millions of German civilians just as the Germans did in the Soviet Union. Once they recovered from the destruction of war, the Soviet Union would then have been a constant threat to France and the rest of Western Europe. The United States also would not have had the opportunity to rebuild Germany as they did in 1945.