The infamous event that marked the beginning of World War II in 1939 was the invasion of Poland by Nazi Germany on September 1. This aggressive act prompted Britain and France to declare war on Germany on September 3, effectively starting the conflict. The invasion employed blitzkrieg tactics, combining fast-moving and coordinated attacks by air and ground forces, allowing Germany to quickly occupy Polish territory. This event was a significant escalation of tensions in Europe and was rooted in the unresolved issues from World War I, leading to a widespread and devastating conflict that would engulf much of the world over the next six years.