Maliki fully understands the scope of this threat and has responded to it directly, hailing the impending withdrawal of U.S. troops as a victory for Iraq—his Iraq—a demonstration that he has achieved true sovereignty, that he can provide security, and that he is not a puppet of foreign occupiers after all.
The stakes are high, precisely because the fight, from this point on, is his, not ours. This, too, is by Maliki's own choice. He has even decided to put the question of America's withdrawal to a popular referendum—knowing that Iraqi voters will approve it by a wide margin.
It is worth recalling that this withdrawal is part of the renewed Status of Forces Agreement, signed by Maliki's government and George W. Bush's administration in November 2008, which allowed the United States to keep any troops in Iraq at all. Article 24 of the agreement—which was drafted at the Iraqis' insistence—requires not only "all U.S. combat forces" to "withdraw from Iraqi cities, villages, and towns … no later than 30 June 2009," but also "all U.S. forces" (not just "combat forces") to "withdraw from all Iraqi territory, water, and airspace no later than 31 December 2011."
If the Bush administration had not agreed to these terms, the SOFA then in place would have expired and the United States would have had to pull out all of its troops and equipment immediately, as they would no longer have had a legal right to be there. (Similar agreements govern the rights and restrictions of U.S. military forces in all foreign countries where they are stationed.)
Some military analysts have speculated that at some point Maliki will feel the pressure and revise the SOFA to let U.S. troops remain. However, this scenario is unlikely, not just politically but legally. Article 30 of the SOFA states that either party can notify the other that it's terminating the agreement—but also that the termination wouldn't take effect until one year after the notice is declared. The Iraqi parliament could theoretically draft a new SOFA, but if the country is falling apart—the premise of this scenario—it's unlikely that the warring factions would agree on a revision, especially one that allowed U.S. troops to stay (and therefore continue to prop up Maliki's regime).
For better or for worse, there isn't much we can do about this situation, however it develops. If the Iraqi government—which we helped make sovereign—wants us to leave, then we'll leave, and so it should be. The SOFA is Obama's inheritance, but it's a convenient one. Without the withdrawal from Iraq, he wouldn't have enough troops to reinforce the fight in Afghanistan—or enough money to finance his domestic agenda.
This situation need not have come to pass. In 2007, then-President Bush laid out a set of 18 "benchmarks" that the Maliki regime had to meet. Most of them involved settling political differences between Shiite and Sunni factions—coming up with formulas for sharing oil revenue, parliamentary power, and so forth. It was a sound idea. The U.S. troop surge, combined with Gen. David Petraeus' counterinsurgency strategy, had gone a long way toward reducing violence. However, as Petraeus often said, this strategy was only a means to an end; the security zones it created at best provided a "breathing space" for Iraq's factions to get their act together and forge a political order.
The benchmarks might have been a way to nudge the Iraqis into this political settlement. Meet the benchmarks (a few at a time), and we'll lavish you with rewards; fail to meet them, and we'll mete out punishments. But Bush didn't follow through; he took no action, one way or the other; the benchmarks had no meaning, and so they were ignored.
By the time the SOFA expired, Maliki's best option for demonstrating his strength, and staying in power, was to insist on a timetable for U.S. withdrawal—unattached to conditions of political progress.
Who knows what will happen over the next 18 months as the U.S. military steadily lowers its profile to the vanishing point. Iraq might erupt in civil war, or Maliki (or someone else) might manage to clench and hammer an iron fist. (The least likely scenario is the Bush/neocon dream of a Western-style democracy taking hold along the Euphrates and spreading like wildfire across the Middle East.)
Either way, formally and practically, it's out of our control.