Which event is often cited as the beginning of World War II?

The invasion of Poland is identified as the beginning of World War II due to its significance in triggering a series of events that escalated into a global conflict. On September 1, 1939, Germany invaded Poland using a military strategy called Blitzkrieg, which involved rapid and coordinated attacks combining air power and ground forces. This invasion breached the Treaty of Versailles, which had placed restrictions on German military expansion after World War I.

The immediate consequences of the invasion were profound. Britain and France, reacting to German aggression and honoring their commitments to protect Poland, declared war on Germany on September 3, 1939. This declaration marked the transition from localized conflicts in Europe to a full-fledged world war as it led to the involvement of numerous other nations in the ensuing conflict.

In contrast, the other events listed do not have the same direct and immediate connection to the outbreak of World War II. The assassination of Archduke Franz Ferdinand is often cited as a catalyst for World War I, the signing of the Treaty of Versailles was a post-World War I treaty that created resentments leading to World War II, and the Battle of Britain occurred later in the war as the United Kingdom defended against the German Luftwaffe. Thus, the invasion of