The United States is now the thread that binds Iraq, and it is clear that a serious unraveling of the situation would occur were this thread suddenly to be pulled away.
In other words, instead of making it easier for us to leave Iraq, the Surge has made it more difficult. And if that doesn't qualify a military tactic as a failure, I don't know what does.
I'm not sure that the surge wasn't in retrospect a deliberate attempt to make it all but impossible for the US to leave Iraq any time soon. And less out of a genuine security worry, than in order to save face for Bush and Cheney. But the whole gambit - and the underlying mission creep - leads to an obvious paradox of history:
As the glue that holds Iraq's disparate parts together, the U.S. is now playing the role that Saddam Hussein formerly played in Iraq, and we're playing it for the same reason that we were willing to tolerate Hussein for as long as we did: to contain Iran's regional influence. What's more, it's a role that has once again led us to ally ourselves with some unsavory and unpredictable characters, all of whom have their own agendas that don't always correspond to ours. And short of the improbable appearance of an Iraqi strongman in the the structural, functional (and moral) image of Saddam Hussein, it's a role that only we can play.