In the Anschluss, Hitler took over Austria because he believed that all German people should be united.
Anschluss is a German word that, in a political context, means "union". It was used to refer to the merger of Austria and Nazi Germany into a single nation, on March 12, 1938, including Austria as a province of the III Reich, passing from Österreich to Ostmark. This situation lasted until May 5, 1945, when the Allies occupied the German province of Ostmark. The Allied military government ended in 1955, when the new State of Austria was formed.
Hitler's motivation for annexing Austria was that he regarded the Austrians as German people living outside their nation. Therefore, the annexation of Austria was an obligatory step towards the attainment of the "Greater Germany".