The significant global conflict that took place between 1939 and 1945 was World War II, which involved many of the world's nations divided into two opposing military alliances: the Allies and the Axis Powers. Sparked by the invasion of Poland by Nazi Germany in September 1939, WWII was a total war that led to unprecedented levels of destruction and loss of life. Major battles occurred across Europe, Africa, Asia, and the Pacific, encompassing key events like the D-Day invasion and the dropping of atomic bombs on Hiroshima and Nagasaki. The war reshaped international relations and resulted in the establishment of the United Nations.