The major international conflict that occurred between 1939 and 1945 was World War II. This global conflict involved numerous countries, primarily divided into the Allies and the Axis powers. It began with the invasion of Poland by Nazi Germany in September 1939 and expanded to include battles across Europe, Asia, and the Pacific. Major events included the Holocaust, the bombings of Hiroshima and Nagasaki, and significant battles like Stalingrad and Normandy. The war ended in 1945 with the unconditional surrender of Germany in May and Japan in September, resulting in the reorganization of international relations, the establishment of the United Nations, and the beginning of the Cold War.