Robert Farley says:
Indeed; to the extent that the United States must devote years, billions upon billions of dollars, and hundreds of thousands of troops to "winning" in Iraq, the very purpose of the invasion is undermined. It does no good to "throw some little country against the wall" if in doing so our own capacity to act is severely wounded; other little countries that might have been intimidated take note of the fact that we are incapable of acting. This was, of course, why Don Rumsfeld bitterly resisted proposals to go into Iraq with substantially more troops, why he resisted the idea of increasing troop levels, and why he resisted the shift to counter-insurgency; he understood that such moves undermined the purpose of the invasion in the first place. To the extent that the war has been about the extension of American imperium, it has failed disastrously.
I would just emphasize that bit about Rumsfeld. To the extent that post-2006 tactics have proven relatively successful in stabilizing Iraq, this does not provide a viable tactical implementation of the strategy encapsulated in Bush's preventive war doctrine. That strategy requires that it be possible to subdue medium-sized countries in a sufficiently easy and uncompromising way that we could credibly threaten to do it over and over again.
What we've seen in the "surge" era is that not only is stabilizing an Iraq-sized country extremely difficult, but also that having any measure of success requires you to really lower the horizons for success. One of the most successful things we've done since General Petraeus took over was simply make peace with groups we were formerly trying to subdue. And of course the fewer people you try to fight the lower your costs get, but also the range of objectives you could achieve gets narrower. Bushism was founded on the presumption that we could accomplish a great deal, with ease, through the use of force unrestrained by law or institutions and it failed miserably.