What event marked the beginning of World War II in Europe on September 1, 1939?

The event that marked the beginning of World War II in Europe on September 1, 1939, was Germany's invasion of Poland. This military action initiated a broader conflict, as it violated the Treaty of Versailles and crossed a crucial threshold that led to widespread warfare in Europe. The invasion prompted Britain and France, who had pledged to support Poland’s sovereignty, to declare war on Germany shortly thereafter on September 3, 1939.

While the signing of the Nazi-Soviet Pact was significant as it secured a non-aggression agreement between Germany and the Soviet Union and included secret protocols regarding the division of Eastern Europe, it did not start the war itself. Similarly, the annexation of Czechoslovakia was a prelude to the war and showcased the aggressive expansion of Nazi Germany, but it did not trigger the global conflict. The declaration of war by Britain followed the invasion and was a direct result of Germany's actions, but it was the invasion of Poland that directly marked the beginning of hostilities in Europe.