The fate of the entire U.S. enterprise in Iraq now hangs in the balance, as the war has entered a dangerous new phase. It is the phase of barbaric identity-card violence between Sunnis and Shiites. In the late 1970s, I covered a similar moment in Lebanon, and the one thing I learned was this: Once this kind of venom gets unleashed — with members of each community literally beheading each other on the basis of their religious identities — it poisons everything. You enter a realm that is beyond politics, a realm where fear and revenge dominate everyone's thinking — and that is where Iraq is heading.

Jeffrey Gettleman reported last Sunday in this paper about Mohannad al-Azawi, a quiet Sunni pet shop owner in Baghdad who was abducted from his store and found murdered the next morning. His skin was covered with purple welts, and his face and legs had drill holes in them. His brother Hassan, the story noted, "carries the autopsy photos with him, along with a pistol. 'I cannot live without vengeance,' he said."

Once embedded, this cycle of fear and revenge is almost impossible to break. People conclude that the only thing that can protect them is a militia from their own sect, not the police or the army. Then these militias, which come to life to protect the neighborhood, take on a life of their own. They develop protection rackets, feel the thrill of power and, as that happens, start to do all they can to prevent the government from restoring its authority. Finally, as the BBC noted in a recent report from Baghdad, some Iraqi politicians are now concluding that "they can gain more power and influence from building on sectarian loyalties than from appeals for national unity." When politicians decide they can get ahead by appealing more to fear than to hope, national reconciliation goes up in smoke.

A Baghdad blogger, the Mesopotamian, quoted by AndrewSullivan.com, gave a vivid description of his neighborhood: "The confusion and conflict between the Americans, the army and the Ministry of Interior is producing a situation where the citizens don't know anymore whether the security personnel in the street are friends, enemies, terrorists or simply criminals and thieves. Everybody is wearing the same uniforms. Whole sections of the city have virtually fallen to gangs and terrorists, and this is especially true for the 'Sunni'-dominated neighborhoods. People and businesses are being robbed and the employees kidnapped en masse in broad daylight and with complete ease as though security forces are nonexistent, although we see them everywhere.