All of the answers saying Poland are incorrect. Before he invaded Poland, in 1939, Hitler invaded Czechoslovakia to reclaim the Sudetenland.
At the Munich Conference, the other powers in Europe decided to support the invasion, agreeing the territory was rightfully Germany’s. Hitler took the Sudetenland and then all of Czechoslovakia, installing a fascist government to rule newly independent Slovakia.
On 15 March 1939, German troops marched into Czechoslovakia. They took over Bohemia, and established a protectorate over Slovakia. Hitler's invasion of Czechoslovakia was the end of appeasement:
It proved that Hitler had been lying at Munich.
It showed that Hitler was not just interested in a 'Greater Germany' (the Czechs were not Germans).
On 17th March, Chamberlain gave a speech saying that he could not trust Hitler not to invade other countries.
On 31st March, Chamberlain guaranteed to defend Poland if Germany invaded.