World War II started on September 1, 1939, when Nazi Germany invaded Poland. The war ended on September 2, 1945, with the official surrender of Japan. To give a bit more detail:

• Germany's invasion of Poland on September 1, 1939, triggered the war.
• Britain and France declared war on Germany shortly after, on September 3, 1939.
• The war in Europe ended in May 1945, with the German armed forces surrendering on May 7 (effective May 8, known as V-E Day).
• The war in the Pacific concluded on September 2, 1945, with Japan signing the surrender documents, marking the official end of World War II.