The significant event in 1939 that marked the beginning of World War II in Europe was Germany's invasion of Poland on September 1. This act violated multiple treaties and prompted Britain and France to declare war on Germany shortly afterward. The invasion was executed using a new military strategy called Blitzkrieg, or "lightning war," which involved rapid and coordinated strikes by air and ground forces to overwhelm the enemy. This marked the start of a global conflict that would engulf many nations and lead to significant political, social, and economic consequences across the world.