World War II began in 1939. It started on September 1 when Germany invaded Poland, leading Britain and France to declare war on Germany shortly thereafter. This conflict involved many of the world’s nations, including major powers such as the United States, the Soviet Union, and Japan, and lasted until 1945. It was marked by significant events including the Holocaust, widespread destruction, and critical battles across Europe, Asia, and the Pacific. World War II ultimately reshaped global geopolitics, leading to the establishment of the United Nations and the beginning of the Cold War, profoundly influencing international relations in the post-war era.