The major global conflict that occurred from 1939 to 1945 is World War II. This devastating war involved most of the world's nations and was marked by significant battles and events, including the Holocaust, the use of atomic bombs, and the rise of totalitarian regimes. The conflict was primarily driven by aggressive expansions from Axis powers like Germany, Japan, and Italy, which faced off against Allied forces. World War II reshaped international relations, leading to the establishment of the United Nations and setting the stage for the Cold War. Its impact continues to be felt globally today.