Another event that almost certainly hastened the Allied victory in World War II was Hitler's decision to commit his last substantial body of reserves to an attack on the Allied forces in the West rather than the invading Soviet armies in the East. The Ardennes offensive of December 1944, the so-called "Battle of the Bulge," was at best a very temporary and transient success, and it burned through the last German reserves of armor and aircraft. In retrospect, it would have probably prolonged the war if the Germans had used these resources, especially their most advanced aircraft, against the Russians, whose air arm was less developed than that of the Allies.
The most decisive single event, however, remains the use of the atomic bombs on Hiroshima and Nagasaki. This gave the Japanese a face-saving excuse to surrender, without which they would undoubtedly have fought to the finish in the home islands, with enormous loss of life.
The decision by Truman to use the atomic bombs on Hiroshima and Nagasaki is generally credited with bringing an Allied victory over Japan more quickly than a mainland invasion might have have produced, while also sparing Allies the enormous number of casualties that surely would have resulted in such an invasion. An additional motivation may have been the cooling of the relationship between the U.S. and the Soviet Union as conflicts began to fester over the division of post-war Europe; some have speculated that Truman saw this as an opportunity to show the Soviets what the U.S. had achieved and possibly intimidate them into being a bit more cooperative.