Which event marked the beginning of World War II?

The invasion of Poland by Germany on September 1, 1939, is widely recognized as the event that marked the beginning of World War II. This invasion signified a pivotal moment in history where Germany, under Adolf Hitler, violated the Treaty of Versailles and attacked a sovereign nation, leading Britain and France to declare war on Germany shortly thereafter.

This action escalated tensions that had been brewing in Europe due to numerous factors, including unresolved issues from World War I and the aggressive expansionist policies of the Nazi regime. The invasion not only demonstrated Germany's military might but also showcased Hitler's ambitions for territorial expansion, fundamentally altering the landscape of international relations and leading directly to a widespread conflict that would engulf much of the world.

The other events listed, while significant in their own right, did not initiate the actual conflict of World War II. The Treaty of Versailles essentially set the stage for dissatisfaction and future aggression, the assassination of Archduke Franz Ferdinand triggered World War I, and the establishment of the United Nations occurred after the conclusion of World War II, aiming at preventing future conflicts. Therefore, the invasion of Poland is recognized as the definitive starting point of World War II.