The major 20th-century war that saw the involvement of over 30 countries between 1939 and 1945 is World War II. This global conflict was primarily fought between the Allies and the Axis Powers, leading to significant changes in the geopolitical landscape. Factors such as militarism, nationalism, and unresolved issues from World War I contributed to the outbreak of the war. Notable events include the Holocaust, the use of atomic bombs in Hiroshima and Nagasaki, and the establishment of the United Nations post-war, aiming to prevent future conflicts and promote international cooperation.