Final answer:
In October 1939-April 1939, Adolf Hitler's aggressive expansionist campaigns threatened the security of Europe, eventually leading to the outbreak of World War II.
Step-by-step explanation:
In October 1939-April 1939, significant events occurred that set the stage for World War II. During this period, Adolf Hitler's aggressive expansionist campaigns threatened the security of Europe. In 1939, Germany invaded Poland, which led to the outbreak of the war. The international tensions and conflicts that occurred during this time marked a crucial turning point in world history.