was the incompetence of its military leaders, particularly Robert E. Lee. Yes, Lee was a brilliant tactician, but his notion of "offensive defense"--which had him invading the North twice, both times with catastrophic results (Antietam, Gettysburg)--was simply absurd under the circumstances. Time and again he risked losing everything in a single battle, and escaped an earlier final defeat only because of the timidity of McClellan at Antietam, and Meade's unwillingness to try for a killing blow after Gettysburg.
Compare Lee's strategy to Washington's during the Revolution. By contrast, Washington avoided battles of attrition, avoided confrontations where all might be lost, rarely went on the offensive, traded territory for time, and with the odds against him far greater than those against Lee, managed to do just what you said--drag out resistance long enough so that the British finally just said, "the hell with it."
Of course, Washington benefited from foreign intervention--the French--and from the fact that Britain was engaged in other wars simultaneous to the Revolution--but the French only intervened because they saw Washington had a good chance of winning. The South might have been able to suck in foreign support--IF it had been willing to give up slavery early on. But that's a whole other discussion.
Just my armchair analysis here, is all.
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