You can say that Germany's aggressive expansion started at home.  It began with the following steps.

• Hitler breaks the Treaty of Versailles by instituting a military draft and creating an air force.
• He remilitarizes the Rhineland where Germany was no supposed to have troops.

From there, Hitler went abroad.  These steps included:

• Helping the fascists in the Spanish Civil War.  He took no territory here, but he was helping an ally take Spain and he was getting practice for his military.
• The Anschluss with Austria.
• Taking the Sudetenland.
• Taking the rest of Czechoslovakia.

After that the next step was to invade Poland, which was the act that finally set off WWII.