In late 1938 he advanced into the Sudetenland, which was the German speaking region of the new country of Czechoslovakia, which had been created after the First World War.
In 1939, Hitler would attack Poland to protect the Germans in the free city of Danzig.
Earlier in the 1930s, Hitler had seized a German speaking province between France and Germany, and he also occupied Austria, which had voted to merge with Germany in a plebiscite.
Anyone who read Mein Kampf, would have known that Hitler never thought Czechoslovakia should have been a country in the first place, it was the "bastard child" of the Versailles Treaty, only there for Britain and France to keep Germany in check. That's pretty much how he felt about all of the countries that were spawned by the breakup of the Austro-Hungarian Empire after the war. Much like how Putin goes on about the breakup of the Soviet Union. Now perhaps Hitler had some valid points there, as does Putin, so the question is, what rights does a nation have to rectify the past? At the time, many did think Hitler had some justification for the Sudenten Germans, but they didn't understand what Hitler's true intentions were, although he pretty much laid them out in Mein Kampf, years before. So what of Putin? Does he really want to rebuild the Russian Empire? Or is truly Ukraine the only hill he's willing to die on?