The Allied Nations did not form an alliance and then go to war. Each side ended up at war with Germany (Britain declared war on Germany after the nation invaded Poland in 1939, and Germany invaded the Soviet Union in 1941 in part to create "living space" for the German people). The United States naturally allied with Britain and supported its war effort from the start. Then, in 1941, after German ally Japan attacked the United States, Germany declared war on the United States in response. So there you have it: the Soviet Union ended up at war with the same German nation as the United States and Britain.