On December 7, 1941 the Japanese navy performed a surprise attack on the navy base at Pearl Harbor, Hawaii. In terms of success, the Japanese wiped out basically everything the United States had stationed at Pearl Harbor. 350 aircraft were destroyed or damaged. All 8 battleships of the Pacific fleet were sunk or badly damaged. Same result for 3 cruisers, 4 destroyers, 1 minelayer, and 5 other auxiliary ships. Amazingly though, all but three of those ships were repaired and returned to duty. Luckily for the United States, none of our aircraft carriers were present in Pearl Harbor.
World War II officially came to an end on September 2, 1945. That's when the surrender was signed, but Japan announced its surrender on August 15, 1945. That was shortly after the atomic bombs were dropped on the cities of Hiroshima (August 6) and Nagasaki (August 9).
May 7, 1945 is the date of the German surrender from World War II, but it was the Japanese surrender that brought all of World War II to a close.
If you are asking for a single defining event that stopped World War II, I don't think it can be done. The Allies and the Soviets surrounding Berlin and Hitler committing suicide are important events to stopping the war in Europe. Dropping the atomic bombs were instrumental in forcing the surrender of Japan in the Pacific.