The major event that marked the official start of World War II in 1939 was the invasion of Poland by Germany on September 1. This act of aggression prompted Britain and France to declare war on Germany on September 3, effectively beginning the conflict in Europe. The invasion showcased Germany's military strategy of blitzkrieg, or lightning warfare, emphasizing speed and surprise. The swift and overwhelming nature of this attack caught Polish defenses off guard, leading to a rapid conquest. This event signified not only the outbreak of a global war but also the failure of diplomatic efforts to maintain peace following the unresolved tensions from World War I.