World War II (1939–1945) was a global conflict involving the Axis powers (led by Germany, Italy, and Japan) and the Allied powers (including the U.S., U.K., Soviet Union, and China). Sparked by Hitler's invasion of Poland, the war saw major battles across Europe, Asia, and the Pacific, alongside atrocities like the Holocaust. Key turning points included D-Day (1944) and the atomic bombings of Hiroshima and Nagasaki (1945). Over 70 million people died, making it the deadliest war in history. The war reshaped geopolitics, leading to the Cold War, decolonization, and the founding of the United Nations. Its legacy continues to influence global politics and collective memory today.