If it wasn't for the U.S. giving an incredible amount of supplies to Britain and winning the battle of the Atlantic Britain would have starved into submission. The Battle of Britain was a multi week air war waged over Britain that was only meant to soften them up for an imminent invasion of the island. Russia was able to win because Germany was fighting a two front war and could keep up the man power to fight the massive soviet army without sacrificing the western front. If we didn't enter the war, Japan would have attacked Russia and then they would have been fighting the entire axis power after Britain had been defeated.