Interestingly, you can trace “American” English becoming its own thing [1] to the publication of the first Webster dictionary that made a conscious effort to simplify spelling. American and British dialects have been diverging ever since (although British has been becoming more American since the turn of the millennium).
Not accurate. Webster didn't originate the American dialect; it had been developing since the English first settled here.
Webster provided a common reference for the American dialect: he helped unify the dialect. Or partly unify: "Redd up your room annat" isn't comprehensible in most American cities, but is still proper English in Pittsburgh. (Of course, nowadays better dictionaries probably list "redd" and "annat", even if they're not used across all of the US.)
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