What event initiated the Second World War in 1939?

The initiation of the Second World War in 1939 is marked by the invasion of Poland by Germany. This event took place on September 1, 1939, when German forces launched a surprise attack on Poland. This invasion broke the terms of the Treaty of Versailles, which had limited Germany's military capacity after World War I, and it demonstrated the aggressive expansionist policies of Adolf Hitler and the Nazi regime.

The invasion prompted Britain and France to declare war on Germany on September 3, 1939, in response to their obligations to support Polish sovereignty. This marked the beginning of the conflict that would escalate into a global war involving many nations. The significance of this event lies in its role as the catalyst for widespread military action and the formation of alliances, thus marking the official start of the Second World War.

Considering the other events listed, while they are significant in the context of World War II, they do not mark the onset of the conflict. The Japanese attack on Pearl Harbor occurred later, in December 1941, and triggered the U.S. involvement in the war. The signing of the Treaty of Versailles occurred in 1919, which ended World War I, and while it certainly impacted the conditions leading to World War II