Is there any correspondence indicating a particular point in time that an Allied general (or leader) was pretty sure of overall victory? Or, perhaps, what was the earliest Allied military/political action that appeared to anticipate a victorious outcome?
If relevant, were the leaders in the U.S. sure of it before those in the U.K.? Or the other way around?
Possible events that come to mind as having enough power to make them believe that victory was inevitable:
-U.S. entering the war (could it have been this early?) -The initial successes of Operation Overlord -Winning the Battle of the Bulge -VE-day
When the United States entered the war was certainly a turning point, but I don't think the Allies believed victory was necessarily there's right away. Remember, the Allies still had to infiltrate mainland Europe to get to Germany, which was no easy feat considering most of the troops were draftees with little to no combat experience. I think most historians point to the time after the Allies broke out of France and started pushing German troops back is a point when they thought the war was going to be won by the Allies. On the pacific front, it was the Battle of Midway that turned Japan back from advancing and toward defending. Hope this helps!
On the day of Pearl Harbour, Churchill wrote in his diary "there was no more doubt about the end... I went to bed and slept the sleep of the saved and thankful" (See http://loc.gov/exhibits/churchill/wc-sword.html). Note that this was actually before Germany (and Italy) declared war on the US (and a few people thought the US would stay out of the European war until they declared war).
Churchill's actions from then on assumed Allied victory.