Gallia est omnis divisa in partes tres. -- Caesar

WASHINGTON -- It took political Washington a good six months to catch up to the fact that something significant was happening in Iraq's Anbar province, where the former-insurgent Sunni tribes switched sides and joined the fight against al-Qaeda. Not surprisingly, Washington has not yet caught up to the next reality: Iraq is being partitioned -- and, like everything else in Iraq today, it is happening from the ground up.

1. The Sunni provinces. The essence of our deal with the Anbar tribes and those in Diyala, Salahuddin and elsewhere is this: You end the insurgency and drive out al-Qaeda and we assist you in arming and policing yourselves. We'd like you to have an official relationship with the Maliki government, but we're not waiting on Baghdad.

2. The Shiite south.This week the British pulled out of Basra, retired to their air base and essentially left the southern Shiites to their own devices -- meaning domination by the Shiite militias now fighting each other for control.

3. The Kurdish north. Kurdistan has been independent in all but name for a decade and a half.

Baghdad and its immediate surroundings have not yet been defined. Despite some ethnic cleansing, the capital's future is uncertain. It is predominantly Shiite, but with a checkerboard of Sunni neighborhoods. The U.S. troop surge is attempting to stabilize the city with, again, local autonomy and policing.

This radically decentralized rule is partition in embryo. It is by no means final. But the outlines are there.

The critics at home, echoing the Shiite sectarians in Baghdad, complain that an essential part of this strategy -- the "20 percent solution" that allows former-insurgent Sunnis to organize and arm themselves -- is just setting Iraq up for a greater civil war. But this assumes that a Shiite government in Baghdad would march its army into the vast Anbar province where there are no Shiites and no oil. For what? It seems far more likely that a well-armed and self-governing Anbar would create a balance of power that would encourage hands-off relations with the central government in Baghdad.

As partition proceeds, the central government will necessarily be very weak. Its reach may not extend far beyond Baghdad itself, becoming a kind of de facto fourth region with a mixed Sunni-Shiite population.

Nonetheless, we need some central government. The Iraqi state may be a shell but it is a necessary one because de jure partition into separate states would invite military intervention by the neighbors -- Turkey, Iran, Saudi Arabia and Syria.

A weak, partitioned Iraq is not the best outcome. We had hoped for much more. Our original objective was a democratic and unified post-Saddam Iraq. But it has turned out to be a bridge too far. We tried to give the Iraqis a republic, but their leaders turned out to be, tragically, too driven by sectarian sentiment, by an absence of national identity and by the habits of suspicion and maneuver cultivated during decades in the underground of Saddam's totalitarian state.

All this was exacerbated by post-invasion U.S. strategic errors (most importantly, eschewing a heavy footprint, not forcibly suppressing the early looting, and letting Moqtada al-Sadr escape with his life in August 2004) and by al-Qaeda's barbarous bombing campaign designed explicitly to kindle sectarian strife.

Whatever the reasons, we now have to look for the second-best outcome. A democratic unified Iraq might someday emerge. Perhaps today's ground-up reconciliation in the provinces will translate into tomorrow's ground-up national reconciliation. Possible, but highly doubtful. What is far more certain is what we are getting now: ground-up partition.

Joe Biden, Peter Galbraith, Leslie Gelb and many other thoughtful scholars and politicians have long been calling for partition. The problem is how to make it happen. Top-down partition by some new constitutional arrangement ratified on parchment is swell, but how does that get enforced any more than the other constitutional dreams that were supposed to have come about in Iraq?

What's happening today on the ground is not geographical line-drawing, colonial style. We do not have a Mr. Sykes and a Mr. Picot sitting down to a map of Mesopotamia in a World War I carving exercise. The lines today are being drawn organically by self-identified communities and tribes. Which makes the new arrangement more likely to last.

This is not the best outcome, but it is far better than the savage and dangerous dictatorship we overthrew. And infinitely better than what will follow if we give up in mid-surge and withdraw -- and allow the partitioning of Iraq to dissolve into chaos.