Between the fall of France in June 1940 and Hitler's invasion of the Soviet Union in June 1941, the British Isles stood completely alone without a major European ally in the struggle against Nazi Germany. Even when the Soviet Union was brought in to the war in far off eastern Europe, it was by no means certain that an alliance between Britain and the Soviet Union would last for more that a few months before the Soviets would be overrun and defeated, such was the initial success of 'Operation Barbarossa'. It was only with the entry of the United States in to World War Two, following the Japanese attack on Pearl Harbor in December 1941, that the ultimate path to victory over Nazi Germany became clearer.