When did World War II begin? For Europeans, it was September 1st, 1939, when Hitler''s Nazi Regime invaded Poland. For Americans, it was on December 7th, 1941, when the Japanese Imperial Navy attacked Pearl Harbor. However, the earliest battle was on August 13th, 1937, in Shanghai, China. Beginning the Second Sino-Japanese War, the Battle of Shanghai raged for 3 months and left over 200,000 dead.