Hitler had already gone against the Treaty of Versailles and shown aggression by taking over Austria. When he Invaded Poland Britain was no longer willing to give them another appeasement and declared war.
Battle of Stalingrad
Italy, Germany, and Japan decided to form an alliance and become the axis powers. They agreed to not interfere with each other's expansion plans and to fight against the Soviets. This also allowed them to make bolder moves against the opposing nations.
Invasion of Italy
The attack on Pearl Harbor by the Japanese led to the United States declaring war against the axis forces. Before they were neutral and didn't supply any weapons but their involvement in the war resulted in a boost of supplies for Britain and the Soviet Union
In 1942 Hitler decided to attack the Soviet Union and take control of their oilfields. They were only able to make it to Stalingrad but were forced to surrender when winter came. This was a huge defeat for Nazi Germany. It took a toll on their troops and equipment.
The alliance decided to travel across the Mediterranean to Sicily then southern Italy after taking over North Africa. The British and American troops were successful in defeating Italian forces and overthrowing Mussolini. This also allowed the Allies to take further action in invading France.
The D-Day invasion was an attempt to regain control over France in 1944. Nazi troops were too weak to keep the allies back and the Allies were successful in their mission to seize Paris and all of France.