The Treaty of Versailles faltered to heal the bitter mess that formed between countries in World War I. It left Germany in a terrible position and gave them a desire for dictatorship. Germany had been ordered to disarm its military and put strict rules on when and how the Germans could rearm. In 1931, Adolf Hitler came to power in Germany. He later rejected the treaty and establish military conscription. Mussolini also became dictator of Italy, while this was occurring. He decided to invade Ethiopia in 1935. Since Ethiopia had lesser power than that of Italy's, they became under complete Italian control.
The news of Germany's rearmament soon reached France. Hitler then became interested in joining the Union of Soviet Socialist Republics in order to protect its security. He then pushed his plan for lebensraum and annexed Austria by force in 1938. Following this, Hitler threatened Czechoslovakia, ordering persecution of the German minorities there. Hitler and Mussolini agreed to the Germans occupation of Sudentenland in September 1938. Then in March 1939, peace broke down when Hitler conquered the rest of Czechoslovakia. He soon afterward started to make orders to Poland, but they resisted at every turn. Notwithstanding the conflict with Britain and France, Hitler decided to invade Poland. In return, Great Britain and France declared war on Germany on September 3, 1939.
Hitler made an announcement to start a pact with the Soviet Union. As the Germans occupied Poland, the Soviets invaded the eastern part of the country with plans to take Latvia, Estonia, and Lithuania in 1940. They also took Finland in 1941 and disgrace the Russian military.
Japan was also changing things in the Pacific. They had plans to conquer China and expand the Japanese Empire into southeast Asia. As this was happening, the Germans took on an approach of blitzkrieg, or lightening war. Since the Germans had no old weapons to deal with, they could easily outfit their troops with the best of weapons.
Hitler then attempted to gain air control over the British Royal Air Force and prepare for an invasion, but the British successfully defeated the German air forces. Overturned with his downfall to take Britain, Hitler turned to the Soviet front, but was defeated as well in 1942.
In 1941, the Japanese thought it was the right time to expand into Greater East Asia. The attacks on Pearl Harbor and the Philippines brought the United States into the war and turned the war for the Allies.