Anschluss - Nazi Germany invades Austria
12 March 1938, Nazi Germany invaded Austria and proceed with the "Anschluss", that is the annexion of that territory to Germany. Four years before, Hitler had ordered the assassination of Austrian Chancellor Engelbert Dolfuss and already attempted to annex that country but failed partly due to the opposition of Mussolini who was not yet his ally at that stage.
When the alliance with Italy became a reality in 1936, nothing stood in the way of Hitler from reuniting the German speaking populations of Germany and Austria. The Austrian people seem to welcome their new leader.
After all, Hitler was one of them. He was born in Austria in Brannau Am Inn, a town that borders Germany and then he grew up in Vienna. It is only after serving in the first war that he actually moved to Munich in Germany. The annexion of Austria violated the treaty signed after WWI. Most of Europe turned a blind eye...