Hitler wanted to join all Germans living outside Nazi Germany together to form a self-sufficient Greater German Reich. To get his hands on sufficient farmland and raw materials, he had to expand in an easterly direction. He saw these areas as Lebensraum.
From the moment Hitler gained control in Germany, he started violating the Treaty of Versailles by building an army and conquering territory. Hitler established an alliance with Italy. They were called the axis powers. During the Conference of Munich Britain and France gave in to his treaty violations and territorial demands to prevent a new war. This policy was called appeasement. The United States weren't involved yet as they had declared a policy of isolationism after the First world war.