The global conflict that began in 1939 and involved most of the world’s nations is World War II. Triggered by the invasion of Poland by Nazi Germany, the war quickly expanded as alliances formed between the Axis powers (Germany, Italy, Japan) and the Allied powers (including the United States, the Soviet Union, and the United Kingdom). The conflict lasted until 1945, resulting in unprecedented loss of life, widespread destruction, and significant geopolitical changes. Post-war outcomes included the establishment of the United Nations, the beginning of the Cold War, and the reorganization of national borders in Europe and Asia.