It matters what we call things. It took too long for the Bush administration to admit that its intended liberation of Iraq had become an occupation, that US forces faced a home-grown insurgency there, and that a transition to Iraqi democracy might not result in a nation that supports US interests.

Finally, not until 2007 did the Pentagon acknowledge that Iraqi sectarian violence had crossed a threshold to become a civil war.

But policymakers still haven't come to terms with the implications of that fact. If they did, they'd see that a wisely executed withdrawal of US-led forces could well be the surest path to peace. That's because withdrawal is likely to transform the fighting in Iraq into a defensive struggle for power in a nation-state, as opposed to an offensive battle rooted in religion.

The evidence overwhelmingly suggests that the war in Iraq is a religious civil war and that – even putting aside Al Qaeda in Iraq – Islam is at the heart of it for three reasons.

First, Iraq's Sunnis and Shiites themselves see the war in these terms. They identify first and foremost as Shiites and Sunnis. Second, they use religious identity both to target opponents and define threats. Finally, they have appealed beyond the borders of Iraq for aid – fighters, arms, cash – in religious terms.

Islam is not based in a specific territory; it is a transnational faith that unites its community, or umma, in the minds of men.

Further, Islam does not have one leader who can dictate what is right or who is wrong. The absence of an ultimate authority figure means that Shiites – who, unlike Sunnis, believe that religious scholars are needed to help interpret the will of God – often latch on to charismatic imams.

This helps explain why the cleric Moqtada al-Sadr has recently committed himself to further religious study in Iran. It also helps to explain why Iraqi Prime Minister Nouri al-Maliki will fail to gain acceptance as a leader among the vast majority of Iraq's Shiite population.

Not only does Mr. Maliki not have support in the street – his government's failure to deliver even basic security and life's needs is apparent to most Iraqis – but he has no religious credentials of his own to fall back on.

By contrast, Mr. Sadr's ability to deliver security and services through his Mahdi Army, and his authority as cleric and the son of the martyred Grand Ayatollah Mohammed al-Sadr, has assured him a devoted following.

Sectarian conflict in Iraq was previously limited to fighting between Sunnis and Shiites. But today, the conflict has grown to include Shiites against fellow Shiites. Despite signs that security has improved, the religious civil wars in Iraq may have only just begun.

My research on civil wars from 1940 to 2000 highlights three important facts about such wars, all of which apply to Iraq. First, nearly half of all ongoing civil wars (46 percent) involve religion in some form. Second, Islam has been involved in more than 80 percent of all religious civil wars. Third, religious civil wars are less likely to end in negotiated settlement. Instead, combatants tend to duke it out until one side achieves victory.

In Iraq, a negotiated settlement is going to be very difficult for two reasons. First, the Shiites will want to remain in almost complete control due to two entirely legitimate concerns: (1) fears of Sunni repression as experienced in the past, and (2) a sense of majority-rule justice. Second, the Shiites themselves are divided on how Iraq should be ruled, so it's difficult to know whom to bargain with on the Shiite side, and therefore who can credibly commit to abide by the terms of any settlement.

What then can the United States and its allies do to bring about a negotiated settlement? Ironically, the best way to support a negotiated settlement would be to leave Iraq.

The withdrawal of US forces would allow Iraq's predominantly Arab Shiites and Sunnis to find common interest in opposing their two more classical historical adversaries: Kurds and Persians. The longer the US and Britain stay, the more they facilitate a shift away from the identity that long unified Iraq to the religious identity that is tearing it apart and facilitating its manipulation by Iran.

There are three obvious downsides to this approach.

First, the end of violence in Iraq following a US withdrawal would lead to the emergence of a nonsecular, nondemocratic government in Iraq. It would be more friendly toward Iran (though not Iran's puppet, as currently feared), but less friendly toward Israel, although a democratic Iraq would be no improvement in this regard.

Second, since US withdrawal has been conditioned on a de-escalation of violence in Iraq, the Bush and Brown governments would be left the unenviable task of explaining to their countries that "withdrawal is the best way to create the conditions for, withdrawal."

Third, withdrawal before violence has fully ceased will look like failure to most Americans and Britons.

The idea of victory versus failure is really a false dichotomy, however. The real choice for US and British policymakers is between the more costly failure that will obtain from current policy and the less costly failure that might obtain from a well- thought-out and well-executed withdrawal.

• Monica Duffy Toft is a professor of public policy at Harvard's Kennedy School of Government.