1. The vast size of the Soviet Union. The Soviet Union was a vast country, with a population of over 200 million people. This meant that the Germans had to spread their forces over a very large area, making it difficult to concentrate their efforts and achieve breakthroughs.
2. The weather. The weather in the Soviet Union was extremely cold and harsh, especially during the winter months. This made it difficult for the Germans to move their troops and supplies, and also contributed to the high rates of sickness and desertion among German soldiers.
3. The Soviet people's resistance. The Soviet people were fiercely patriotic and determined to defend their country from the German invaders. They fought bravely, even when they were outnumbered and outgunned.
4. The mistakes of the German high command. The German high command made a number of mistakes during the invasion of the Soviet Union. These included underestimating the strength of the Soviet military, failing to adequately plan for the harsh weather conditions, and allowing the Soviet troops to retreat and regroup.
5. The Allied support for the Soviet Union. The Allies provided the Soviet Union with vital military and economic aid during the war, which helped the Soviets to turn the tide of battle and eventually defeat the Germans.
In conclusion, Hitler's invasion of the Soviet Union was unsuccessful due to a combination of factors, including the vast size of the Soviet Union, the harsh weather conditions, the fierce resistance of the Soviet people, the mistakes of the German high command, and the Allied support for the Soviet Union.