In my opinion Operation Barbarossa was the most crucial turning point in the war. Although Operation Sea Lion and Pearl Harbor had their own significant contributions, Operation Barbarossa managed to weaken Germany's moral, their forces, and split their army into two fronts. It is to my belief that Germany's two front war is the main reason they lost in the first place. Hitler invaded the USSR with all intentions of taking of abruptly, however, with the combined efforts of Russian defense, Mother Nature, and overall Russian nationalism they did not succeed. The USSR used a method called the scorched earth tactic. This is where they retreated their forces but destroyed anything that could be used by the Germans alone the way such as food, resources, and railroad lines. By doing this they slowed Germany's overall offensive attack causing them to fight during the winter. This is where Mother Nature took its toll in Germany's failure. When the hard Russian winter fell the German soldiers were not ready. They were dressed in light green camouflage. First off it was extremely cold so most soldiers froze to death and second the green camouflage was very easy to pick out on the white snowy terrain making it impossible for the soldiers to hide. Combine the scorched earth tactic and Mother Nature with overall Russian Nationalism and will to survive made Germany's offensive attack turn defensive and caused a crucial turnaround in the war. .
After Germany's misguided failure in trying to take London, their moral was already down going into the USSR. Leaving Great Britain alone and only wounded to go "pick a fight" with the USSR allowed Great Britain to re-gather their forces and eventually initiate another attack on Germany. This mistake led to a split in the German forces and an eventual loss in the war. All three turning points throughout the course of the war coincided with each other and caused Germany's defeat.