Final answer:
Germany took aggressive action in the 1930s by annexing Austria and Czechoslovakia, which demonstrated their expansionist ambitions and foreshadowed the outbreak of World War II.
Step-by-step explanation:
The aggressive action that Germany took in the 1930s was annexing Austria and Czechoslovakia. Germany, under the leadership of Adolf Hitler, sought to defy the Treaty of Versailles and reunite all German-speaking peoples. Hitler's troops moved into Austria without facing resistance on March 12, 1938, in what was known as the Anschluss.
Later that year, at the Munich Conference, Hitler was granted the Sudetenland region of Czechoslovakia through appeasement. Despite the agreement, in March 1939, German forces went on to occupy the rest of Czechoslovakia, revealing the futility of appeasement. These actions demonstrated Germany's expansionist ambitions and set the stage for World War II.