This is worth reading. "Petraeus's experience over the past decade shows that generals are not immune to the strain. They get used up, too," writes Greg Jaffe. I have heard Petraeus' tour in Afghanistan referred to as his "fat Elvis" period, an unkind way of saying that he was tired and not on his A-game there.
I suspect that in Iraq, Petraeus tried to work with local factors as points of leverage, while in Afghanistan, his campaign against corruption worked against local factors. But I don't know the Afghan war well enough to assemble the evidence for that hunch. Can anyone enlighten me on this?