Does the conflict in Iraq amount to a civil war? In many ways, the public debate over this question is largely political. Calling Iraq a "civil war" implies yet another failure for the Bush administration and adds force to the question of whether U.S. troops still have a constructive role to play.

Politics aside, however, the definition of civil war is not arbitrary. For some -- and perhaps especially Americans -- the term brings to mind all-out historical conflicts along the lines of the U.S. or Spanish civil wars. According to this notion, there will not be civil war in Iraq until we see mass mobilization of sectarian communities behind more or less conventional armies.

But a more standard definition is common today:

1) Civil war refers to a violent conflict between organized groups within a country that are fighting over control of the government, one side's separatist goals, or some divisive government policy.

By this measure, the war in Iraq has been a civil war not simply since the escalation of internecine killings following the bombing of a Shiite shrine in Samarra in February, but at least since the United States handed over formal control to an interim Iraqi government in June 2004.

Here's why: Although the insurgents target the U.S. military, they are also fighting the Shiite-dominated Iraqi government and killing large numbers of Iraqis. There is little reason to believe that if the United States were suddenly to withdraw its forces, they would not continue their battle to control or shape the government.

Political scientists who study civil war have proposed various refinements to this rough definition to deal with borderline cases. One issue concerns how much killing has to occur -- and at what rate.

2) For a conflict to qualify as a civil war, most academics use the threshold of 1,000 dead, which leads to the inclusion of a good number of low-intensity rural insurgencies.

Current estimates suggest that more than 25,000 Iraqis have been killed in fighting since the U.S.-led invasion in March 2003 -- a level and rate of killing that is comparable to numerous other conflicts that are commonly described as civil wars, such as those in Lebanon (1975-1990) and Sri Lanka (beginning in 1983).

The organization -- or rather, disorganization -- of the warring communities in Iraq means that a large-scale conventional conflict along the lines of the U.S. Civil War is unlikely to develop. More probable is a gradual escalation of the current "dirty war" between neighborhood militias that have loose ties to national political factions and are fighting almost as much within sectarian lines as across them.

This is roughly what happened in Lebanon and at a lower level in Turkish cities in the late 1970s. Ethnic cleansing will occur not as a systematic, centrally directed campaign (as in Bosnia), but as a result of people moving to escape danger.

And there's another twist to the terminology:

3) If the conflict in Iraq becomes purely a matter of violence between Sunni and Shiite communities driven by revenge and hatred rather than by political goals, many political scientists would say that it is something other than civil war.

Almost no one, for example, calls the Hindu-Muslim violence in India a civil war.

A civil war has to involve attempts to grab power at the center of government or in a given region, or to use violence to change some major government policy.

In Iraq's case, however, the vacuum of power at the center means that communal violence will inevitably be tied to struggles for political power and control.

A final complication concerns the nature of international involvement. Some argue, for example, that the war in Bosnia should be seen as an interstate war rather than a civil war, since the Bosnian Serb forces were armed and directed largely by Belgrade. Post-Mobutu violence in Congo is often termed a civil war, even though fighters have been closely tied to armies from neighboring states.

4) A conflict may be both a civil and an interstate war at the same time.

The Vietnam War, for instance, clearly comprised both a civil war in the South and an interstate war involving the North, the South and the United States.

Iraq may be moving in this direction. The United States and Britain are already openly involved, and such neighboring countries as Iran and Syria are more covertly involved. Not that it matters to the people dying there, but the next debate here may turn on whether what is already a civil war in Iraq should be viewed as an interstate war as well.

Before attacking Iraq in March 2003, the United States told the world that Saddam Hussein had reconstituted his nuclear weapons program in defiance of the United Nations. That claim, used to justify the war, was based on assessments provided by the United States intelligence community. But as everyone now knows, those assessments were wrong. So Jeffrey T. Richelson's history of American nuclear intelligence, including our attempts to learn about Iraq's nuclear program, could hardly be more timely.

In "Spying on the Bomb," Richelson, the author of several books on American intelligence, has brought together a huge amount of information about Washington's efforts to track the nuclear weapons projects of other countries. He examines the nuclear projects of Nazi Germany, the Soviet Union, China, France, Israel, India, South Africa, Taiwan, Libya, Pakistan, Iran and North Korea, as well as Iraq. Through interviews and declassified documents as well as secondary works, he sets out briefly what we currently know about those projects and compares that with assessments of the time.

This may sound like heavy going, but Richelson writes with admirable clarity. And along the way he has fascinating stories to tell: about plans to assassinate the German physicist Werner Heisenberg during World War II; about discussions in the Kennedy and Johnson administrations on the possibility of attacking Chinese nuclear installations; about Indian measures to evade the gaze of American reconnaissance satellites; and about the bureaucratic infighting over the estimates on Iraq.

The United States has put an enormous effort into gathering information about the nuclear projects of other countries. After World War II it equipped aircraft with special filters to pick up radioactive debris from nuclear tests for isotopic analysis. It created a network of stations around the world to register the seismic effects of nuclear explosions. Most important, in 1960 it began to launch reconnaissance satellites that could take detailed photographs of nuclear sites in the Soviet Union and China. Richelson occasionally speculates about the role of communications intercepts and of spies, but these appear from his account to have been much less important than the other methods of collecting information.

Through these means the United States has gathered a vast quantity of data, sometimes to surprising effect. Intelligence played a crucial role in the cold war, for instance, by reducing uncertainty about Soviet nuclear forces. Alongside such successes, however, there have been failures. One notable example concerned the first Soviet test, which took place in August 1949, much sooner than the C.I.A. had predicted. Another was the failure to detect Indian preparations for tests in May 1998, even though at an earlier time the United States, with the help of satellite intelligence, had managed to learn about preparations the Indians were making and to head off their tests.

But the most serious failure of all was in Iraq in 2003, because in no other case did the intelligence assessments serve as justification for the use of military force. The information needed for avoiding political surprise is one thing. That needed for preventive war is quite another, if only because of the consequences of making a mistake.

Beyond making the uncontroversial recommendation that "aggressive and inventive intelligence collection and analysis" should continue, Richelson draws no general conclusions. That is a pity, because his rich material points to issues that cry out for further analysis. He suggests in one or two cases that failures sprang from the mind-set of the intelligence community, but he does not elaborate on this point. He has little to say about relations between policy makers and the intelligence community, even though the quality of intelligence and the use made of it depend heavily on that relationship.

His focus is no less narrow in his discussion of foreign nuclear projects. He concentrates on the programs themselves, paying very little attention to their political context. Does that reflect a technological bias in nuclear intelligence? Would, for example, the prewar assessment of Iraqi nuclear capabilities have been more accurate if it had paid more attention to the broader political and economic circumstances of Hussein's regime?

The task of intelligence has become more complex than it was during the cold war. A single dominant nuclear opponent has now been replaced by a number of nuclear states, along with states and stateless terrorists that are aiming to get their hands on nuclear weapons. Meanwhile, the technology needed for producing nuclear weapons has become easier to acquire.

Many critics believe the recent performance of the intelligence community shows it has not responded adequately to this new situation. Richelson does not have much to say on this question; nor does he discuss the likely impact of the current reforms, initiated in response to the Iraq war, on the quality of intelligence. His reticence may imply that he does not think reform is necessary. Still, it is disappointing that he does not draw on his historical survey to discuss whether new approaches are needed for dealing with nuclear threats, and, if so, what those new approaches might be.

Noah Feldman

In looking at the brewing civil war between the two groups in Iraq, it's easy to assume that the cause is ancient hatred. Nothing could be further from the truth. For the overwhelming majority of Iraqi history, Sunnis and Shi'ites have lived peacefully side by side, and numerous Iraqis are the children of mixed marriages. Instead we are witnessing in Iraq what occurs when government collapses and there is no state around capable of guaranteeing personal security.

What do you do when your family is in peril and you cannot turn to the government for protection? The answer is that you will take security wherever you can get it. You need to find some group that will be capable of keeping you safe, and that group had better be one that can count on your loyalty just as you can count on its protection. If you are a member of my ethnic, racial or religious group, then we share at least some basic bond, which may be enough to ensure our loyalty to one another. I need some assurance that you will have my back, and identity is better than nothing.

Sunnis and Shi'ites may find themselves joining militias or supporting denomination-based political parties even if they are not particularly pious and would much prefer not to. Something similar happened in the former Yugoslavia when its government collapsed with the fall of communism and nothing replaced it. Ethnic activists - call them identity entrepreneurs - will always form the core of the new militia. These radicals will emphasize symbols, like al-Askari mosque that was blown up last week in Iraq, and hope that followers will react by strengthening their commitments to the group itself.

Is it possible to break the cycle of violence that gets under way when identity groups move toward civil war? One answer is for an outside force to impose a solution. The killing did not stop in Bosnia or Kosovo until Western powers showed they were willing to bomb. But this approach is not viable in Iraq, where U.S. bombs came first and civil strife has followed. Instead the only way out of the violence is for Iraqis to realize that they have more to gain by negotiating a settlement between their groups than they do by allowing a full-blown brothers' war to break out.

Vali Nasr Author The Shia Revival (forthcoming)

What lies at the heart of the sectarian violence in Iraq is not so much religious dispute as it is a very secular competition for power and prominence in the new Iraq. Iraq is not all that different from Northern Ireland or Bosnia, where religion paraded as ethnicity and became a vehicle for communal rivalries. In the vacuum of power left by the fall of Saddam Hussein, the game of numbers has favored Shi'as, who are 60% of the population. It is for this reason that they wholeheartedly embraced democracy. Disgruntled Sunnis, on the other hand, vested their fortunes in boycott and violence, hoping that as spoilers, they would gain leverage in negotiating over the future.

Few in the West recognized the depth of either the Shi'a anger at the Saddam regime or the Sunni rage born of loss of power. There is a strong sense of Iraqi identity among both Shi'as and Sunnis, but as strong allegiance to sect and ethnicity in every election has shown, a shared notion of what Iraqi identity means and how each community sees the future of Iraq is fast disappearing. As happened in Bosnia, in Iraq mixed marriages and shared memory of coexistence will not be enough to stop internecine violence.

Shi'as embraced the political process that the U.S. set in place in 2003 in the hope that it would guarantee their security and serve their interests. There is indication now that many Shi'as are having second thoughts. Already overstretched in facing the Sunni insurgency, the U.S. can hardly afford losing the Shi'a as well. If tensions escalate to a full-blown civil war, Iran, Saudi Arabia, Jordan and Syria may all join the war to protect their co-sectarians and to scramble for pieces of a failed Iraq.

Pulling Iraq back from the brink will be difficult. Building a strong central government and an effective security force will help. The challenge is to get them up and running before events on the ground pass a point of no return.

James D. Fearon

By any reasonable definition, there has been a civil war in progress in Iraq at least since the Coalition Provisional Authority formally handed over authority to the Iraqis in 2004. A civil war is a violent conflict within a country fought between organized groups seeking to compel a major change in government policies or to take control of the center or a region. The insurgents in Iraq target the U.S. military, but they are also fighting against the Shi'ite-dominated Iraqi government and killing large numbers of Iraqis. There is little reason to think that if the U.S. suddenly withdrew, the insurgents would not continue to fight to control or shape the government.

When we hear talk about incipient civil war in Iraq, the fear is of an escalation of the current insurgency into a much bigger war. Analysts may have in mind something like the U.S. Civil War, with Sunni and Shi'ite armies fighting each other across well-defined fronts. Or they may imagine a sudden spasm of massive communal conflict and ethnic cleansing along the lines of Bosnia or Rwanda. Neither scenario is all that likely, although bouts of violent ethnic cleansing are certainly possible in a few parts of the country, especially Kirkuk.

My guess would be that as the insurgency continues to create insecurity, sectarian militias will continue to grow in power and influence. They will increasingly supply local security, but in the form of protection rackets that extort as they protect. They will clash with each other over territory and control of revenue sources. Since the Sunnis remain highly disorganized, some of these local fights may initially be intra-Shi'ite. But in the absence of effective political incorporation and protection from national police and army units - which are heavily infiltrated by Shi'ite militias - Sunnis will gradually form a patchwork of militias. Neighborhood-by-neighborhood conflict and violence will increase. Think Lebanon.

Juan Cole

If you look at the ethnic conflicts and street demonstrations during Iraq's modern history, it is remarkable how few have involved Shi'ites fighting Sunnis. During the colonial era, Iraqis were united by their opposition to the British occupation. Sunni and Shi'ite tribes cooperated in rebelling against British rule, and were only put down with a bombing campaign in 1920 that killed 9,000. In 1941 mobs targeted Iraq's small Jewish population; Jews had been a valued part of the Iraqi national fabric but were accused, unfairly, of being pro-colonial. After World War II, much of the violence in Iraq was fueled by issues of class. In 1948 slum dwellers and railway and oil workers revolted against a government treaty with Britain. In 1959, Arab nationalists assassinated Communist Party members, while mobs in Mosul and Kirkuk attacked and killed rich businessmen and landowners.

Iraqi Muslims have not all along been severely divided by religious sect. There have been many instances of strong cooperation between Sunnis and Shi'ites. Other social divides have led to mob violence in the past, but Iraqis have overcome them to re-establish national unity. It remains to be seen whether they can accomplish this feat again.