On Wednesday, Gen. George Casey, the top U.S. military commander in Iraq, told reporters in Baghdad that Shiite death squads were now openly operating against Sunni civilians. And Prime Minister Maliki told the Iraqi parliament that their country had only one "last chance" left to escape a collapse into full-scale civil war.

In fact,they are already in one.

More violent sectarian clashes were reported Wednesday in Baghdad between gunmen armed with rocket-propelled grenades and police and residents in Um al-Maalif, a mainly Shiite neighborhood in the southern part of the city.

The latest violence followed a rampage by a mob of gunmen believed to be members of Shiite militias through the al-Jihad district of western Baghdad Sunday in which up to 60 people, all or almost all of them Sunnis, were reported killed. People were dragged from their cars and even from their homes at random and shot, the New York Times said.

As we have reported in previous columns, the highest level planners in the Pentagon and the U.S. National Security Council have consistently underestimated the power, independence and capacity for sectarian violence of the Shiite militias in Iraq. U.S. forces patrol as little as possible except when on major search and destroy operations against Sunni insurgents.

Responsibility for daily law and order, especially in Baghdad, has been handed over as much as possible to the new Iraqi Army and police forces as deliberate U.S. policy.

But reliable police forces, as the British Empire's administrators knew in their huge collection of territories around the world, take years and even decades to become experienced, disciplined and reliable.

They cannot be rushed into existence in a few months or a year or two as the United States has tried to do in Iraq. Far from protecting people against the Shiite militias the new Iraqi police have had best proved utterly powerless against them, and at worst appear likely to have been heavily infiltrated by them.

These developments are all too familiar to anyone who has covered the evolution of sectarian conflicts around the world from Northern Ireland and Lebanon to Rwanda.

U.S. policymakers continue to describe the Iraq war as a straightforward "white hats versus black hats" face off between the U.S. armed forces in Iraq and the Iraqi army and security forces America has worked so hard to build up on one side, and the Sunni insurgents led by Islamist extremists and Baath regime veterans on the other.

But since the bombing of al Askariya, the Golden Mosque, in Samara on Feb. 22, the nature of the conflict has completely changed. Shiite militias have been killing Sunnis - often innocent people - at a rate four times higher than the casualties Sunnis have been inflicting on them.

Even worse, the Shiite militias have not been operating independently of the government's new Iraqi armed forces but in many cases have been operating from within them. There are no security screens to prevent militia veterans from being recruited into the Iraqi police and army, which are run by Shiite dominated forces.

What has happened in Iraq since Feb. 22 -- and the conditions for it in fact were remorselessly building up for at least two and half years before that -- is that Iraq has already fragmented into a state of sectarian conflict. This is a condition very familiar to the long-suffering citizens of Belfast and many other cities around the world. And it has unleashed dynamics very different from a straightforward guerrilla revolt operating out of an ethnic minority community.

When Belfast and Beirut rules apply, a society has already fragmented into a mosaic of mutually hostile parts. Effective political power within those parts, or ghetto-ized communities, has already devolved down to the militias who dominate them. That means the militias become the source not only of physical protection, but also the providers or arbiters of public services like sanitation, running water and electrical power.

Once confidence in a central government is shattered, as happened in Belfast in 1969 and in Beirut in 1975 - it may take decades to win it back.

The first priority of any national or international peacekeeping force caught in this kind of situation, whether it be the British Army in 1970s Northern Ireland or the U.S. Army in Iraq today, has to be to get the killing and retaliatory attacks under control and prevent them escalating into an out-of-control bloodbath.

Therefore, what U.S. commanders and their political masters in Washington should have done in recent weeks was quite clear. There was only one way to prevent the escalation of the Iraq conflict to the level of full-scale rampages and massacres of civilians, such as started in Baghdad on Sunday -- and that was by the massive, decisive deployment of far larger forces of reliable troops on the streets in Baghdad as a preemptive measure.

But the United States simply does not have remotely enough troops in Iraq to do that job and the continued attrition inflicted by the Sunni insurgents has generated massive political pressure in Washington to keep U.S. casualties down as much as possible, especially with the midterm congressional elections coming in November. Thus, there was no effective mood made to head off Sunday's massacre. And it is likely that far worst ones will follow.

The massacre of Sunni civilians by rampaging Shiite gunmen Sunday marks a major and exceptionally dangerous escalation in the Iraq conflict. Sunday's massacre indicates that the civil war in Iraq is already far worse than the 30-year Northern Ireland conflict ever was.

It is already comparable in savagery to the Bosnia conflict of 1991-94 that cost up to a quarter of a million lives and to the Lebanon conflcit that erupted in 1975 and cost 100,000 dead over the following decade and a half.

As was the case in Bosnia, ethnic massacres and mass deportations on a scale far larger than any yet seen are now extremely likely in the next few months unless they can be headed off by wise statecraft and/or security measures. Sunday's bloodbath in Baghdad may be only the first taste of things to come.

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