"Although the war began with Nazi Germany's attack on Poland in September 1939, the United States did not enter the war until after the Japanese bombed the American fleet in Pearl Harbor, Hawaii, on December 7, 1941."
"75 years ago, Pearl Harbor attack brought US into WWII. PARIS, France — The December 7, 1941, Japanese attack on Pearl Harbor triggered the United States to join its European allies in World War II."
How it ended:
"World War 2 ended with the unconditional surrender of the Axis powers. On 8 May1945, the Allies accepted Germany's surrender, about a week after Adolf Hitler had committed suicide. VE Day – Victory in Europe celebrates the end of the Second World War on 8 May 1945."