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.

It is time to acknowledge that the international community's strategy on Zimbabwe has failed.

Robert G. Mugabe's regime has survived even as the economy deteriorates further (unemployment is above 70 percent, and gross domestic product will decline another 7 percent this year) and personal freedom suffers greater assaults (the recent "drive out the rubbish" campaign left 700,000 people homeless).

Indeed, with the opposition Movement for Democratic Change (MDC) on the verge of collapse - following a bitter fight over whether to boycott the recent Senate elections and after years of sustained government pressure - the regime has a stronger grasp on power than ever. Doddering though he may be, Mr. Mugabe, who recently turned 82, has foiled the pressure of the United States and Britain and the quiet diplomacy of his neighbors in southern Africa.

Predictions of imminent change still crop up in Western newspapers on the occasion of every new crisis in Zimbabwe. But these predictions have not come to bear, and they likely will not. So long as Mr. Mugabe reigns, his Zimbabwe African National Union-Patriotic Front (ZANU-PF) regime will survive.

The international community tried to change things. It embarked on a strategy of concerted economic and diplomatic pressure to weaken the Mugabe regime, trying to force it to either back down or submit to the democratic opposition. It's clear that strategy has failed.

It is, therefore, time to focus international attention on the prospect of Zimbabwe's only genuine political opening in the years ahead: the exit of Mr. Mugabe, whether through retirement or death, which will leave the regime internally and externally vulnerable.

Internally, the ZANU-PF regime without Mr. Mugabe at the helm will be uniquely susceptible in an election. In sub-Saharan Africa, opposition candidates have won post-transitional elections only 5 percent of the time against incumbents but 33 percent of the time against regimes' designated successors.

The most important reason for that is the incumbent's exit removes the regime's glue. The regime fractures into competing factions and is left with a substantially reduced capacity to repress the political opposition and rig an election.

In Kenya, after President Daniel T. arap Moi, and in Ghana, after President Jerry J. Rawlings, the regimes did not - could not - resort to all the dirty tactics that they certainly would have used had the incumbents run. In turn, these political openings have tended to galvanize the fractured opposition to successfully cooperate.

A ZANU-PF that is deeply unpopular, badly fractured among ethnic groups and between moderates and hard-liners (the expulsion of the information minister, Jonathan Moyo, is the beginning) and facing a reinvigorated opposition will not likely be able to effectively rig elections, let alone win the popular vote.

It will be critical, then, that presidential elections be held within a year of Mr. Mugabe's exit, before the regime has too much time to consolidate. If Mr. Mugabe's exit does not occur within that window before the 2008 elections, then international, and particularly regional, pressure will be crucial in forcing early elections.

Externally, Mr. Mugabe's exit may prompt genuine regional pressure. Analysts have long emphasized that international pressure requires the support of Zimbabwe's neighbors - especially South Africa - that have significant political and economic leverage. But to the great frustration of Western governments, southern African countries have thus far refused to publicly challenge Mr. Mugabe.

Their reluctance has much to do with Mr. Mugabe's status as a hero of Zimbabwe's anti-colonial struggle and a champion of liberation struggles elsewhere. Southern African nations will have much greater political room to apply real pressure on Zimbabwe when its leader lacks such credentials.

Simultaneously, the prospect of an altogether different level of violence might shake the complacency of southern African nations. Zimbabwe's implosion has not, thus far, been entirely bad for its neighbors. They have benefited from the elimination of economic competition and from the influx of professionals, and they have retained confidence that Mr. Mugabe can keep control.

But there is a real danger, if a post-Mugabe Zimbabwe is not handled adroitly, that elements of the opposition, disaffected war veterans and youth militia and losers in the ZANU-PF factional battle will take up arms and plunge Zimbabwe into civil war.

This specter should push neighboring countries to step up their efforts, especially to press the post-Mugabe regime to hold new presidential elections and encourage moderate elements within ZANU-PF.

Notwithstanding its occasional fulminations against Zimbabwe, the United States has failed in its efforts to unseat Mr. Mugabe's regime. The United States should focus now on his eventual exit by helping the MDC to overcome its bitter infighting and engaging Zimbabwe's neighbors, especially South Africa, in vigorous diplomacy, pushing them to prepare for the occasion.

The stakes could not be higher, for if the post-Mugabe period is the first genuine opportunity for political change in Zimbabwe, it may also be the last for some time.

"The United States is the most powerful since the Roman Empire," stated Stephen Walt, the Belfer Professor of International Affairs and academic dean of the John F. Kennedy School, Harvard University, delivering the 2005 Robert G. Wesson Lecture in International Relations Theory and Practice, at the Freeman Spogli Institute on November 16, 2005. America's unmatched power is therefore of great interest and concern to leaders in most other parts of the world, from President Putin in Russia, to President Chirac in France, and President Musharraf in Pakistan. For Americans, however, the key issue is how others are now responding to U.S. power.

Speaking before an audience of Stanford faculty, students, and the broader community, Professor Walt examined three interwoven themes: why other states do not welcome U.S. power; what are the main strategies available to them for dealing with American power? and what should the United States do in response?

As an integral part of his analysis, Walt showed opinion polls demonstrating a striking gap between American views of U.S. primacy and other countries' perceptions of the current U.S. role. For example, although the 2002 Pew Global Attitudes Project found that 79 percent of U.S. citizens believe it is good that "American ideas and customs are spreading around the world," and 70 percent think that U.S. foreign policy takes the interests of other states into account either "a great deal" or "a fair amount," overwhelming majorities overseas say the United States considers the interest of others "not much" or "not at all." Similarly, a 2005 BBC survey of 21 countries found only five, India, the Philippines, Poland, South Africa, and South Korea, where a majority of people had "positive" attitudes toward the United States.

There are three major sources of anti-Americanism, Walt explained. First, our sheer power makes other nations nervous. Second, there is a perceived sense of hypocrisy between our words and our actions. The case of nuclear weapons provides a vivid example. We preach nonproliferation, yet accord new respect and policy cooperation with newly nuclear states, such as India. Third, how the United States behaves in the world-what we do-invites antipathy. This latter point is abundantly clear in global opinion polls: Even in regions where anti-Americanism seems most strident, nations and individuals report that they do not object to our values or to what we stand for but rather to what we do.

Other nations, Walt pointed out, can choose a strategy of accommodation to our power or a strategy of resistance. Commonly adopted strategies of accommodation include 'bandwagoning," or realigning foreign policies with U.S. wishes, such as Libya's abandonment of nuclear weapons; "regional balancing"-using U.S. power to balance regional threats; "bonding" to curry favor with the United States; and "penetration," a strategy aimed at infiltrating the American political system to influence foreign policy outcomes.

In contrast, countries that choose to resist American power pursue five strategies:

"balancing" our power, alone or in alliance with others; "asymmetric responses," such as terrorism, which try to exploit specific areas of U.S. vulnerability; "blackmail," like North Korea's efforts to extract concessions from its nuclear weapons program; "balking," or tacit non-cooperation; and "delegitimizing," or attempts to turn others against the legitimacy of our actions or policies.

In light of the growing antipathy to U.S. primacy in so many parts of the world, Walt proposed three major courses of action to produce a more favorable response to U.S. power. First, he urged that we reduce American's military footprint abroad-and especially our ground force deployments-and return to a more traditional policy of regional balancing in cooperation with other nations. This policy would make greater use of American air and naval power and limit American intervention to cases where vital U.S. interests are threatened. Second, we should work harder to defend our international legitimacy and rebuild the U.S. image abroad, through a sustained campaign of public diplomacy and by keeping key American institutions-such as higher education-available to foreign visitors. Third, he advocated a more nuanced approach to America's traditional support for Israel, one that balances our genuine support for Israel's existence with the urgent need to bring a lasting settlement to the Israeli-Palestinian conflict.

"The more the United States uses its power in an overwhelming and capricious manner," Professor Walt warned, "the more the rest of the world will resist us." Conversely, the more the United States recognizes and respects the interests of others, while using its power to defend its own interests, the more other nations will welcome U.S. power. "The task we face," he advised, "is to rebuild the trust, admiration, and legitimacy the United States once enjoyed, so the rest of the world can focus not on taming U.S. power but on reaping the benefits it can bring."

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.