The event that officially marked the start of World War II in Europe was Germany's invasion of Poland on September 1, 1939. This surprise attack prompted Britain and France to declare war on Germany two days later, signaling the escalation of tensions that had been building throughout the 1930s. The invasion utilized Blitzkrieg tactics, emphasizing swift and coordinated strikes that overwhelmed Poland's defenses. The rapid fall of Poland demonstrated the effectiveness of Germany's military strategy and set the stage for a broader conflict that would engulf much of the world over the next six years.