General Charles Cornwallis was an actual historic figure and not merely a character suggested by a historic figure. The personal journals that Martin captures in the movie were also real and Cornwallis later published them in several volumes. Cornwallis was born into new British nobility and spent all his younger years in noble circles. He purchased a commission in the British Army and then unlike most gentried officers he actually studied for it. He proved to be a successful and competent officer.
As far as the movie goes, Cornwallis was not as old as he is portrayed in the movie. In reality, he was in his early 40's during the war. All indications were that he wasn't as pompous as sometimes depicted in the movie. As a matter of fact, he was known to be popular with his men, because he went down to the battlefield and into danger to encourage his men, rather than simply remaining at a command post far away from danger as depicted in the movie.
Cornwallis mentored Lt. Colonel Banastre Tarleton, the figure on which Colonel William Tavington was based, just as Cornwallis seems to do in the movie a bit with Tavington. They corresponded until Tarleton published his own memoirs in which he blamed Cornwallis for the British loss in America (and you only thought that mudslinging memoirs were a more modern invention). Cornwallis was meticulous and his plan for movement of his forces was much as depicted in the movie.
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