I'd echo this, especially for a Master's thesis. When I compare how ambitious/naive my initial vague idea for a PhD thesis was versus the far narrower topic I ended up actually writing on, the gap is huge. And that's leaving aside the whole issue of history still being written.
I wrote a short undergraduate dissertation several years ago on the same subject, Earl Cornwallis and the failure of Southern loyalism. Be happy to email it to you, PM me if interested.
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