Short answers, pulled from the story.
World War II ran from the 1st of September 1939 to the 2nd of September 1945. Most historians date its start to Germany's invasion of Poland on the 1st of September 1939, with Britain and France declaring war two days later. Japan signed the surrender documents at Tokyo Bay on the 2nd of September 1945.
World War II caused the death of 60 to 75 million people, making it the deadliest conflict in history. Millions died from massacres, starvation, disease, and genocides, including the Holocaust. By the war's end, some 3.3 million Soviet prisoners of war had died in German captivity, a mortality rate near 60 percent.
World War II was fought between two coalitions, the Allies and the Axis powers. The Tripartite Pact united Japan, Italy, and Germany as the Axis powers, later joined by Hungary, Slovakia, and Romania. The Allied Big Four were the Soviet Union, China, the United Kingdom, and the United States.
World War II grew out of unresolved tensions after World War I, the rise of fascism in Europe, and militarism in Japan. The Treaty of Versailles stripped Germany of around 13 percent of its home territory and all its overseas possessions, fuelling revanchist nationalism. The policy of appeasement allowed Hitler to remilitarise the Rhineland and annex Austria and the Sudetenland with little opposition.
World War II ended in the Pacific after the United States dropped atomic bombs on Hiroshima and Nagasaki in early August 1945. Between the two bombings, the Soviets declared war on Japan and invaded Japanese-held Manchuria, defeating the Kwantung Army. Japan surrendered on the 15th of August 1945 and signed the surrender documents on the 2nd of September 1945.
World War II transformed the political, economic, and social structures of the world. The United Nations was created, with China, France, the Soviet Union, the United Kingdom, and the United States becoming permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the Cold War, while Europe's waning powers triggered the decolonisation of Africa and Asia.