Obama said in his speech that between August 2010 and December 2011, the remaining 35,000-50,000 troops—a major reduction from the 142,000 troops in Iraq now—will have three missions: training, advising, and equipping the Iraqi forces (as long as they don't slip into sectarian conflicts); protecting U.S. civilian and military personnel still in-country; and engaging in counterterrorist operations.
It's not entirely clear what this last task entails. Many officers believe that the best way for troops to fight terrorists is through counterinsurgency techniques—living among the local people, earning their trust, and thus gathering intelligence on who the terrorists are and where they're hiding. This is what most U.S. combat brigades have been doing in Iraq since the "surge" began—and, more to the point, since Gen. David Petraeus started applying his counterinsurgency strategy—in early 2007. The withdrawal of combat brigades presumably means the end of this strategy. Or does it? Will combat units smaller than brigades continue the strategy in certain areas? Or will counterterrorism consist simply of bombing and strafing jihadists if and when they're encountered? The details haven't been spelled out.
The president's vision of post-withdrawal Iraq is also a little hazy. In his speech, he talked about continuing to assist Iraq economically, coordinating regional diplomatic efforts to sustain its stability, and serving as an "honest broker" to help settle internal disputes. All this sounds good, and maybe it will be good; I hope so. But it's conceivable that some of Iraq's neighbors want to keep the place a little off-balance, the better to cultivate their own influence. And it's unclear what leverage we would have as an honest broker without any troops on the ground.
There is a still-larger uncertainty along these lines—whether President Obama has an explicit "Plan B." If August 2010 rolls around and Gen. Odierno tells him that some crucial area of Iraq is falling apart and major violence will erupt if the last U.S. combat brigade pulls out, would Obama be inclined to stretch his timetable or even send back some troops whom he'd withdrawn earlier? Gates was asked at his press conference whether the president had discussed this possibility. He replied that the question was "hypothetical" (which it wasn't) and that, in any case, his decision took those risks into account (which sidesteps the possibility that the calculations of risk turn out to be wrong). It's a question that might be worth asking again in one forum or another.
If all hell breaks loose, is it possible to revise the SOFA to let U.S. troops remain? Strictly speaking, no. Article 30 states that either party can notify the other that it's terminating the agreement—but it also notes that the termination wouldn't take effect until one year after the notice (by which time the full withdrawal might be mandatory). The Iraqi parliament could theoretically draft a new SOFA, but the one in place now took many months to compose, and if the country is falling apart—the premise of this scenario—it's unlikely that the factions would agree on a revision or on wanting U.S. troops to stay in any case.
The bottom line is probably this: President Obama simply wants to get out of Iraq. So does 69 percent of the U.S. population (as do, judging from the applause that greeted his announcement, many of the Marines who have fought there in multiple tours). His budget plans depend on a drastic winding down of this war. So does his broader legislative agenda. And if he wants to send substantially more troops to Afghanistan—a decision he hasn't yet made—none are available unless he takes some out of Iraq.
To some extent, his calculations ride on hope. It's hard to imagine that any American wouldn't hope that, in this case, he's right. But, as has been true throughout this war, the Iraqis have a say in this, too. What Iraq is like in 18 months, or three years, will depend above all on the Iraqis.