In 1936, Hitler marched into the land west of the River Rhine. The Allies, avoiding conflict and overestimating Hitler’s military strength at the time, did nothing.
In March 1938, Hitler marched into Austria and was welcomed as a hero. This was known as the Anschluss. The secret police rounded up anyone who resisted. In a referendum, following the Anschluss, 99% of Austrians voted to unite with Germany. The Allies made only verbal protests.
Hitler’s next demand was the area of Czechoslovakia adjacent to Germany - the Sudetenland. By March 1939, he had occupied Prague itself.
Hitler’s next move was likely to be Poland. The Allies opened talks with the Russians, but on 22 August 1939, Hitler announced a Non-Aggression Pact with Stalin. Armed with this guarantee, ten days later, Hitler invaded Poland.