was any kind of military genius.
As has been pointed out, his victories were all won against generals the like of McClellan, Pope, Burnsides. He was defeated, not only by Grant, but by Meade at Gettysburg--this after Meade had only had command of the Army of the Potomac for a few days.
Lee may have been outnumbered and outgunned, but so were the Vietcong, the American revolutionaries, the French/British on the field in 1914. All his victories were won on "home" soil, where he had the advantage of local support (providing intelligence as well as provision), and vastly greater knowledge of the local terrain. Whenever he was without these advantages -- Antietam, Gettysburg -- he lost, and only the timidity of McClellan and Meade kept those defeats from ending the war entirely.
Furthermore, as has also been pointed out, he seemed to have little or no concept of grand strategy, particularly in a defensive war. Gettysburg was the result not only of a tactical mistake (Picket's charge) but a huge strategic blunder. Longstreet, recognizing that the war would ultimately be won or lost in the west, suggested sending a corps from Lee's army to relieve the siege of Vicksburg. Lee opted for an invasion of the north instead.
By contrast, George Washington, who faced far longer odds than Lee, always had a clear vision of what he needed to win: namely, not to lose. His strategy during the revolution was that of a classic (albeit pre-industrial) insurgency. Avoid pitched battles in which your army might be destroyed. Avoid being trapped and besieged. Break your forces down when possible for quick, sharp attacks on favored terrain (i.e., Trenton) to keep the enemy off balance. At all costs, keep your army in being, and keep it mobile. Yes, it's less "glorious" than winning set piece battles like Chancellorsvile (in which Lee's casualties were staggering--making his decision to invade the north even more foolhardy), but at the end of the day you wear your opponent down.
Finally, one of the most important (and unlearned) lessons of the American revolution was that, even with Washington's brilliance, it might well have failed without foreign intervention. Southerners understood this--they fully expected the British and French to intervene. The Emancipation Proclamation didn't necessarily end that hope. All that southerners had to do to win foreign invention in January 1863 was issue their own emancipation proclamation. For me this is irrefutable proof that slavery and racism were at the heart of the confederate cause--no matter what revisionists today might claim. When faced with the choice of ending slavery or losing he war, they chose to lose the war. Not even the argument made by some of the top military command (not Lee, though) that win or lose slavery was dead, swayed southern politicians. Ditto the proposal to offer slaves who would fight for the south their freedom. As one southern newspaper put it
, "If slaves can make good soldiers, then our whole theory of slavery is wrong." Precisely.
The cult of Lee masks all this, and more. Which is why it is so important that 150 years after the war began we still need to debunk it.