GEN. DAVID PETRAEUS, commander of U.S. forces in Iraq, has promised to return to Washington in September to report on the outcome of his surge strategy. I hope he will say that sectarian killings, bombings and U.S. casualties are all down. But even if he does, I doubt he can offer a clear, plausible date by which the Iraqi army and police will be able to stand on their own without massive U.S. support. So regardless of what he concludes, we seem destined to enter the presidential election season with no credible date for a U.S. exit from Iraq.

In more than four years of war, there have been countless turning points at which we were led to expect decisive political progress in Iraq: the capture of Saddam Hussein (December 2003); the turnover of sovereignty (June 2004); elections for the constituent assembly (January 2005); elections to ratify the constitution (August 2005); and elections for the Iraqi parliament (December 2005).

The surge was the last military card we had to play, and now our bluff will soon be called.

In my view, there is only one condition under which we can withdraw from Iraq with our core interests fully protected and with a reasonable claim that our mission was accomplished, and that is when strong Iraqi military and police forces emerge that can operate independently of U.S. forces and prevent a takeover of the country by either Al Qaeda in Iraq, resurgent Baathists or Muqtada Sadr’s Shiite militia.

Let’s not kid ourselves. The situation today is in some ways much worse than the one faced by President Nixon in Vietnam 35 years ago. At that time, South Vietnam had an army with a paper strength of 1 million men that, despite its problems, was able hold on for three years after the U.S. withdrew its ground forces. The South Vietnamese army provided Henry Kissinger with his “decent interval” between the U.S. withdrawal and South Vietnam’s collapse. (Indeed, Kissinger argues with some plausibility that the South Vietnamese military could have hung on indefinitely if Congress hadn’t cut off funds for U.S. air support.)

Nothing like that exists or will exist in Iraq for the politically meaningful future. As of November, the Pentagon claimed it had trained 322,000 Iraqi military and police, but it admitted that the actual number on hand was much lower because of desertions and attrition. Iraqi forces continue to suffer huge shortfalls in armor, weaponry, logistics and communications, and it is unclear how they would fare without American hand-holding.

Serious training of Iraqi forces started late and never received adequate funding or top-level attention, despite the fact that Petraeus was at the helm of the training effort in recent years. The South Vietnamese army may have been nothing to write home about in 1972, but we are extremely unlikely to have an Iraqi equivalent by the end of 2007.

What all this means is that even if the surge, by September, is reducing violence in Iraq to some degree, it will not guarantee a “safe” exit strategy for U.S. forces.

But here’s the problem: Do we have any other choice than to withdraw? We could stick it out, and I suspect that we could avoid losing in Iraq for another five, 10 or 15 years, as long as we’re willing to maintain high troop levels, continue to spend large amounts of money and suffer more casualties. But even the most conservative Republican candidates are unlikely to campaign on a platform of staying in Iraq indefinitely when the primary season starts next winter and the war enters its sixth year.

This means that we will have to engage in a very different debate from the one we have been having up to now, a debate not about surging and not about withdrawing with our goals accomplished but about how to draw down our forces in a way that minimizes the costs that will inevitably accompany our loss of control.

This is a difficult situation, but it is necessary. The questions we need to address include: How do we reconfigure our forces to provide advice, training and support, rather than engaging in combat? How we can withdraw safely without a serious Iraqi army to cover our retreat? How will we dismantle enormous bases like Camp Liberty or Camp Victory and protect the diminishing numbers of U.S. troops in the country? Do we trust the Iraqi military and police sufficiently to turn over our equipment to them? How do we protect the lives of those who collaborated with us? The images of South Vietnamese allies hanging to the skid pads of U.S. helicopters departing Saigon should be burned into our memories.

And what if the weak Iraqi government we leave behind falls or other political crises occur when we have fewer U.S. troops to respond? Can we work with proxies, resources or arms supplies to shape outcomes?

As we draw down, the civil war is likely to intensify, and the focus of our efforts will have to shift to containing it within Iraq’s borders. Preventing intervention by outside forces will become an even more urgent priority.

On the other hand, it is not necessarily the case that the situation will spiral out of control. Although the situation is graver in some ways than Vietnam, in others it is better. Although we have no equivalent to a South Vietnamese army, the enemy has no equivalent of the North Vietnamese army. It is hard to see any of the small factions struggling for power in different parts of the country emerging as a dominant force throughout Iraq.

The presence of U.S. forces has itself been a spur to terrorist recruitment, but as it becomes clear that we are on our way out, it will be easier for Iraqi nationalists to turn against the foreign jihadists (as they have already begun to do in Al Anbar province).

An intensifying civil war will be a tragedy for Iraq, but it is not the worst outcome from a U.S. standpoint to have a number of bitterly anti-American groups duking it out among themselves.

Civil wars eventually come to an end when one side wins (unlikely, in this case) or when the parties exhaust themselves and drop their maximalist aims.

The war is not lost, despite the assertions to that effect by Senate Majority Leader Harry Reid (D-Nev.). But victory is not around the corner either. We need to start figuring out how to leave this zombie-like zone now.