The conflict that began with the invasion of Poland in 1939 was World War II. On September 1, 1939, Nazi Germany, led by Adolf Hitler, invaded Poland, utilizing Blitzkrieg tactics that combined air assaults and fast-moving ground troops. This act of aggression prompted Britain and France to declare war on Germany on September 3, 1939, marking the official start of the war. World War II would expand rapidly across Europe and later involve nations around the globe, becoming one of the deadliest and most widespread conflicts in history, profoundly impacting subsequent international relations and global history.