If the Iraq war was winnable, we would have won by now.
A new article from the Carnegie Endowment points to some of the reasons this is true:
[S]upporters of the war and opponents both know that the multiple conflicts in Iraq have no military solution. Soon to be chairman of the Joint Chiefs, Admiral Michael Mullen, is unequivocal on this: "Security is critical to providing the government of Iraq the breathing space it needs to work toward political national reconciliation and economic growth.… Barring that, no amount of troops in no amount of time will make much of a difference" (emphasis added). If U.S. forces cannot make a difference, improved Iraqi forces certainly cannot.
What, then, is the political and economic situation? Moderate Sunnis have left the government, Shia unity has crumbled, and Kurds and Shia are less, not more, willing to share power with the Sunnis. Seventeen of 38 cabinet ministers have walked out. Former Sunni insurgents have turned against Al Qaeda in Iraq in Anbar province, but this does not mean support for the Iraqi government or for U.S. goals. More and more of the Iraqi people look to a source other than the government (a sectarian party or militia, Islamist terrorists, a tribe, a criminal gang) for the security and services the Baghdad government cannot provide. By the Pentagon’s reckoning, unemployment stands at about 60 percent, draining the economic base any government needs to stand on.
Basra, Iraq's second largest city and not long ago relatively peaceful, is the place where the "clear, hold, and build" strategy the United States is now following was first applied by the British and judged to be the model to follow. Today it is lawless and bloody, in the grip of warring Shia militia and fundamentalist clerics. The International Crisis Group (ICG), whose reporting from Iraq over five years has been among the best, fears that Basra’s fate will be the country's. The Iraq ICG sees is "a failed state—a country whose institutions and, with them, any semblance of national cohesion, have been obliterated."
As convenient as it would be to have a scapegoat, Iraq's political disintegration cannot be blamed on its prime minister, nor fixed by replacing him. It sources are deeper. Minority Sunnis, who ruled the country for a long time, are still unwilling, as the recent National Intelligence Estimate found, "to accept a diminished political status." That hasn't changed in four years and probably will not until they have fought to exhaustion for what they see as their rightful place.
The political disintegration also comes from the momentum of violence. More than 4 million Iraqis are refugees, internally displaced, or dead from violence. In per capita U.S. terms, that would be nearly 50 million people. Could we, under such conditions, come together as a nation, bury past wrongs, and under foreign military dictate reallocate wealth and make frightening political accommodations? The question answers itself—yet we continue to insist that Iraqis can, perhaps if we threaten a bit more.