What was Hitlers big mistake of World War 2?

Hitler's biggest mistake in World War II was the invasion of the Soviet Union in 1941, known as Operation Barbarossa. This decision stretched German resources thin and opened a massive eastern front that Germany was unprepared to sustain, especially during the harsh winter. Additionally, underestimating Soviet resilience and industrial capacity ultimately led to significant German losses and a turning point in the war. This blunder contributed to the eventual defeat of Nazi Germany.