Operation Barbarossa, also known as Operation Fritz, was a German invasion into the Soviet Union on June 22, 1941. Taking the Soviet Union by surprise, as Hitler and Stalin had signed political and economic pacts. Despite being a surprise attack, Russia was able to fend of Hitler's invasion fairly well, and ultimately lead to Germany fighting a two front war against Russia and the Allies.
Let's take a look at what happened. Germany started out with over 3 million soldiers and 3 thousand tanks when they entered the Soviet Union. While they had an immense number of soldiers and weaponry, their weakness was underestimating their opponent. Germany's attack reached all the way into Moscow, where they faced many Soviet counter-attacks. The German troops had to wait for backup and supplies to continue, and when they finally came in September, heavy rain and bad weather postponed further damaging attacks, as the German soldiers were unprepared and ill-supplied for cold weather and winter. Soviet attacks eventually caused them to retreat and end operation Barbarossa in December.
How was this a turning point in the war? Even though it was more of a last resort (due to political differences), Britain and America eventually allied with Russia to help them end Nazi Germany and the war. This would not have been possible without the betrayal of Hitler and Stalin's "alliance". Germany had to now fight a two front war, which put them in a weaker position.
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