Over the next two years and three months there will be two time lines running in Iraq.

One will be that of U.S. troop withdrawal. The other will be developments in Iraq itself, among the Iraqis. It is absolutely critical from the American point of view that the two not be allowed to cross. In other words, it is vital that U.S. troop withdrawal from Iraq not be affected by anything that the Iraqis do, or start to do, to each other.

This is a judgment based on American considerations. The Iraq war was unnecessary. It has now cost the United States more than 4,300 lives and an estimated $700 billion. The financial cost will continue to rise as the United States seeks to refurbish its forces and military equipment, and, more sadly, as it seeks to cope with the damage done to American soldiers by the experience, the physically wounded and those emotionally impaired by one or more tours in Iraq over the past six and a half years.

The countdown for U.S. withdrawal from Iraq has begun. The U.S. troop level now stands at 128,000, down from the 2007 peak during the "surge." On June 30 some U.S. combat troops were withdrawn and, in effect, banned from Iraqi cities without prior Iraqi permission, a bizarre constraint for an occupying army to put on itself. All U.S. combat troops are scheduled to be out by August 2010, with withdrawal commencing in earnest after Iraq's national elections in January. All U.S. troops are scheduled to be out by Dec. 31, 2011.

This withdrawal should take place on schedule, without regard to what happens in Iraq or to what President Barack Obama or the U.S. military perceive as rising needs for U.S. forces in Afghanistan. If the objective is to renew U.S. forces and their equipment, as it should be, that purpose would not be served by shifting them to Afghanistan instead.

The parallel but perforce separate time line taking place inside Iraq will be political and military. It promises to be explosive. It has been already in that two key Iraqi ministries, foreign and finance, were blown up last month. Both attacks showed the inadequacy of Iraqi forces in maintaining the security even of key buildings in Baghdad. In neither case, however, did the Iraqi government call U.S. forces to ride to the rescue. The cavalry stayed on the rim of the hill.

The attitude that the U.S. government should maintain as Iraqis sort out who will rule Iraq after the United States leaves should be clear: The 29 million Iraqis have the right of self-determination, a principle to which Americans attach the utmost importance when it comes to our own country.

The fact that it will be messy in Iraq, reflecting the history of the country and the different peoples who comprise it, is none of our affair and no justification whatsoever for the U.S. delaying withdrawal or for re-intervention. Nor is the fact that the current structure of the new, post-Saddam Hussein Iraq is in no small part our doing since the invasion of 2003. U.S. withdrawal from Iraq will take place for our own reasons, just as our initial attack on the country did.

But it will be messy. The ethnic and religious make-up of Iraq will lie at the basis of the problems, becoming more intense as the time for full U.S. withdrawal ticks down. The population is approximately 60 percent Shiite Muslim, 20 percent Sunni Muslim and 20 percent Kurdish. Each group wants to rule the country, or at least part of it. And one prize that comes with political power is access to the lion's share of the country's oil revenues, big money.

The United States, on the praiseworthy although naive basis of seeking to institute rule in Iraq on the basis of "one person, one vote," was instrumental in putting the country's Shiite Muslim majority in power, in the form at present of the government of Prime Minister Nouri al-Maliki. It has furthermore trained and equipped Iraq's security forces, which are predominantly Shiite, not surprisingly since the government is predominantly Shiite.

The first problem will be the Sunnis. Even though they are only 20 percent of the population the Sunnis ruled Iraq from independence in 1932 until the United States got rid of Saddam Hussein, a Sunni, in 2003. The Sunnis at first resisted the American occupation strenuously, but then figured out that there was no point in dissipating their forces and energy in fighting people who would be leaving eventually anyway. Instead, at least some of them, the so-called Sunni Awakening, signed on with the United States to help restore order and armed themselves on that basis. There is no reason to believe that the Sunnis will acquiesce in Shiite rule after having run the place themselves for 70 years until the United States turned up.

The other problem is the Kurds. Iraqi Kurds, always in quest of a homeland along with their fellow Kurds in Iran, Syria and Turkey, have had a field day with the Americans ostensibly in control in Iraq. They have been America's best friend since after the first Gulf War. During the occupation they have become increasingly accustomed to ruling their own end of Iraq in cooperation with the Americans, pretty much without interference from the Shiites in the Baghdad government or the Sunnis, who brutally suppressed them in the Saddam Hussein days. Fortunately for them, if there were any justice in Iraq they would benefit from the oil that lies in some of the territory they consider theirs. Unfortunately for them, in Iraq justice doesn't enter into it. Military force and the willingness to use it carry the day.

Again, the trick for America will be to stay out of intra-Iraqi combat as it picks up as we head for the door. Mr. Obama will need strong nerves and the will to resist cries of alarm, including from America's own military-industrial complex.