: A Brief Introduction to World War II Adolf Hitler's invasion of Poland in September, 1939, ignited (点燃) World War II. One by one, Europe’s nations fell before Germany’s mechanized armies 机械化军队 . Only Britain held firm 坚守着 . While the allies (同盟国) conferred (协商) , treachery (背叛) approached from the East. On December 7th, 1941, Japanese planes attacked Pearl Harbor 珍珠港 . The next day, the United States entered the war. Fighting continued for almost four more years, spreading to North Africa and the Pacific. On D-Day, June 6, 1944, Allied troops began the liberation of Europe. Within a year, Germany surrendered 投降 . To subdue (使屈服) Japan, the United States dropped atomic bombs 原子弹 on Hiroshima 广岛 and Nagasaki 长崎 . Japan formally surrendered on September 2, 1945. World War II claimed more than 27 million lives and caused 33 enormous human suffering.