This whole Transaction [General Braddock's loss] gave us Americans the first Suspicion that our exalted Ideas of the Prowess of British Regulars had not been well founded. (3.100)
In retrospect, this is a really tongue-in-cheek thing to say, as Franklin's writing about this part of history so many years later, after Americans revolted against the British and overcame them. It's pretty sarcastic, in hindsight, to make fun of these "exalted ideas" as things without foundation. It also seems, in context with other problems Franklin has coming up against British government officials, that this isn't, in fact, the first sign of trouble.