Lee's defining weakness was using the aggressive battle tactics that worked for him in the Mexican War in the Civil War despite the development of rifled muskets, massed artillery, and defensive works that made such tactics too costly to sustain in the long run. Lee's genuine talent and the fighting spirit of his men won quite a few remarkable battles, but in the end he bled the South dry and failed to convert battle victories into a winning strategy.
Yet Lee deserves great credit for refusing Jefferson Davis's order to engage in guerrilla warfare. Lee's surrender at Appomattox was his single best decision and his greatest service to the country.
"Lee's defining weakness was using the aggressive battle tactics that worked for him in the Mexican War in the Civil War despite the development of rifled muskets, massed artillery, and defensive works that made such tactics too costly to sustain in the long run."
Yep. Lee stuck with Napoleonic tactics much to long after it had become apparent that they had become obsolete. Malvern Hill and Fredricksburg should have made it abundantly clear that Pickett's Charge was a fools errand. Longstreet tried to warn him but he wouldn't listen.