Enlarge By Ali Al-Saadi, AFP/Getty Images The remains of a car bomb that targeted a police patrol in Baghdad Thursday. The latest bout of bloodshed has undermined a massive security crackdown in Baghdad, but President Bush says pulling U.S. troops out of Iraq now would be a "major defeat."

The reality is that the United States is flailing as Iraq hovers between a low-grade civil war and the full-blown version. Most of today's violence is among rival religious sects; the insurgency linked to al-Qaeda terrorists, which ignited the sectarian violence, trails well behind.

That reality leads to a hard truth: The United States probably has one last shot at achieving minimal stability in a unified Iraq. Bush's rhetoric aside, that is the best outcome that can be reasonably hoped for at this point.

Accomplishing it means preventing an all-out civil war long enough that Iraq's fledgling government — which has been in office barely three months — has a decent chance of taking control, particularly of the proliferating sectarian militias. If that can't be done, it's hard to see a useful role for U.S. forces.

Certainly, it's difficult to know just when the line into full-scale civil war is crossed. Bosnia in the 1990s provides a useful yardstick. The point of no return there came when extremists managed to stir passions to the point at which neighbors felt they had no choice but to turn on their neighbors. That hasn't yet happened in Iraq, where polls show most Iraqis crave a normal life.

Their hopes are now concentrated on the "Battle of Baghdad," a massive new security operation in which U.S. and Iraqi troops are going door-to-door looking for militia members, protecting potential victims and persuading shops to stay open. The idea is that if security can be achieved in the capital, where Shiites and Sunnis are more closely intermingled than anywhere else, the civil war threat will ease. It is having some success.

Whether it will work over the longer term is anyone's guess. Nonetheless, a stable Baghdad is critical. The city is home to one-fifth of the population, as well as the central government and news media. What happens there sets the tone for the rest of the country.

Bringing enduring calm, though, will require more than the temporary infusion of troops the United States has poured into the city. The Iraqi government will need to disband the militias affiliated with various factions, ranging from radical Shiite cleric Muqtada al-Sadr's Mahdi Army to private forces linked to different ministries.

It's worth giving the fledgling government time to try to do that.

Iraqi Deputy Prime Minister Barham Salih told us on Thursday that in his most optimistic scenario, a political consensus might emerge within six months, with a good number of armed insurgents persuaded to lay down their arms. His pessimistic scenario: a continuation of the present sectarian violence.

Last week, the savvy U.S. ambassador to Iraq, Zalmay Khalilzad, who has been instrumental in moving the political process forward, said the Battle of Baghdad "will determine the future of Iraq, which will itself go a long way to determining the future of the world's most vital region."

That strikes us as a good measure for judging whether the mission in Iraq can continue to be salvageable.