What event marked the start of World War II?

The invasion of Poland in 1939 marked the beginning of World War II as it was a direct military aggression by Nazi Germany and its allies, triggering a series of military responses from other countries. On September 1, 1939, German forces invaded Poland from the west, while Soviet forces invaded from the east on September 17. This act of aggression violated international treaties, particularly the Treaty of Versailles, which had attempted to prevent another large-scale conflict. The invasion of Poland led to declarations of war from France and the United Kingdom, who had pledged to protect Poland's sovereignty. Consequently, this pivotal event set off a chain reaction of military actions and alliances, ultimately engulfing much of the world in a destructive and catastrophic war.