Six Months That Could Change IraqThree possible post-surge scenarios.

Thomas Friedman has been lambasted for writing way too many times (at least 14 times from late 2003 to mid-2006) that "the next six months" will determine the future of Iraq. The waggish blogger Atrios, who first cataloged this habit, coined the term "Friedman Unit," defined as six months in the future.

That said, the next six months might really be the decisive six months in the course of this war—and, as much as recent trends seem hopeful, the long-term prognosis, alas, doesn't.

In July 2008, the last of the surge troops—the five extra brigades that President George W. Bush sent to Iraq as a last-ditch effort to impose order—will come home, having finished their 15-month terms of deployment. No troops will be sent to replace them (mainly because, for all practical purposes, no such troops will be ready to go). The surge, in short, will be over.

At that point, things will likely go in one of three directions—one good, two bad.

The first scenario is that Iraq's security forces will have ramped up, in quality and quantity, to the point where they can effectively step in where U.S. forces have stepped out—and that Iraq's political leaders will have settled their sectarian disputes. If both of those things happen, Iraq's future will look not quite bright but not so gloomy, either.

If those developments don't occur, then one of two entities might start spiraling toward collapse—the Iraqi nation or the U.S. Army.

Is the surge working? That depends on what you mean by "working." Initial skeptics of the surge (and I count myself, as well as the entire Joint Chiefs of Staff, among them) never disputed the notion that more troops, shrewdly deployed, would impose order and reduce violence where those extra troops were deployed. We doubted whether a mere 20,000 extra troops could impose order throughout Iraq (or even in all Baghdad) on a sustained basis.

There was also the larger point, emphasized by the U.S. military commander, Gen. David Petraeus, himself—that there is no strictly military victory to be had in Iraq. The point of the surge was to create "breathing space" that might allow Iraq's political leaders to reconcile their differences and form a viable central government. If the Iraqis didn't take advantage of this breathing space, the most brilliant strategy and the most successful operations would ultimately be for naught.

And so, yes, violence is down; casualties—American and Iraqi—are down. This is indisputably the case, and one factor in this is certainly the surge and, as important, the counterinsurgency strategy that Petraeus has crafted to go with it.

But there are other factors, as well, especially the alliances of convenience that Sunni tribes have struck with U.S. forces—most notably in Anbar province—to beat back the jihadists of the al-Qaida in Mesopotamia organization. These deals were struck, at the Sunnis' initiative, before the surge began. American commanders are also paying other Sunni insurgents not to attack their soldiers (a wise use of money). And, while this has been going on, Shiite militia leader Muqtada Sadr has been observing a moratorium on attacks against "coalition forces."

Another factor is that, as a result of the past two years' ethnic cleansing, much of Iraq, especially Baghdad, is now Balkanized. Sunnis have killed or chased away most Shiites in Sunni areas, and vice versa.

However, there are still vast swaths of mixed neighborhoods throughout Iraq, especially in Baghdad. And it is widely and firmly believed—by military commanders and seasoned reporters on the ground—that these areas would erupt in horrendous bloodshed if American troops were not clamping down on the fault lines.

Now here's the problem: All these phenomena—the surge, the alliances of convenience, the moratorium—cannot last forever. In fact, they can't last much longer than (pardon the phrase) the next six months, maybe (let's make it two F.U.s) the next year.

First, the troops. Not only will five of the current 20 brigades be out of Iraq by July, but Secretary of Defense Robert Gates and the Army chiefs would like to cut troop-deployment terms from 15 months back to their normal 12 months (with 12 months back home for rest and further training). The 15-month terms were always seen as a temporary measure. Talk to soldiers, and they will tell you that 15 months of continuous combat duty is simply too wearing. Enlistment rates are down; junior officers are dropping out at rates unseen since Vietnam days. The war in Iraq is the main reason for both trends. The Army is already stretched to the bone. Senior officers—and Gates—are deeply worried that maintaining this breakneck pace for much longer might break the all-volunteer Army.

However, if the terms of duty are cut back to 12 months of deployment, followed by 12 months home, it is not clear whether even 15 brigades can be sustained in Iraq for very long. And once troop levels fall below 15 brigades, it is not clear—as they approach 10 brigades, it is very unlikely—that the mission of securing the Iraqi population (the essence of counterinsurgency) can be sustained.

The clock is also ticking on the other games that are keeping ultraviolence at bay. After the Sunni-U.S. alliances defeat the jihadists, or reduce their ranks to a manageable level, nobody expects the Sunni fighters—who, before their "awakening," spent much of their time shooting and blowing up American soldiers—to become pliant citizens. (Stalin didn't join NATO or the IMF after he and the Western allies beat Hitler, either.) They will go back to shooting our soldiers, undermining the Shiite-led Iraqi government, or both; in fact, having gained the experience of fighting alongside U.S. troops, and the armaments that went with it, they will be a more formidable force in sectarian battles with Shiites.

If the Sunni insurgents resume their sectarian battles, it is doubtful that Sadr's Mahdi Army will maintain its cease-fire.

In sum, U.S. forces may soon have more eruptions to damp down—or, to switch metaphors, more holes in the Iraqi dike to plug up. And the task will be more daunting still once the troop-levels decline.

All this is why Gen. Petraeus and most other officers refrain from wild cheering at the reports of declining casualties and violence. They are pleased with the results; they have done what they set out to do, more creatively and successfully than many critics had predicted (and, I confess, I count myself among that number, too). However, they know, first, that these trends can be reversed in a flash and, second, that they mark only tactical successes in this sort of struggle—that the true, strategic signposts of success are, ultimately, beyond their control.

And that's why, this time around, the next six months might really be crucial.