N the spring of 2003 the Bush administration sent Noah Feldman to Iraq to advise American occupation authorities and the Iraqis on constitution making. The choice was remarkably apt, for Feldman possessed a rare blend of talents. A young and respected professor of constitutional law at New York University, he spoke and read Arabic fluently and held a doctorate in Islamic studies. Nor was his the normal Bush appointee's resume. A self-described liberal Democrat, Feldman had clerked for Associate Justice David Souter and litigated for Al Gore in the Florida ballot melee in 2000.

Feldman's most important quality, however, may have been his deep belief in the compatibility of Islam and democracy. He belongs to a small but growing movement among scholars of Islam, a group diverse enough to include Gilles Kepel of France and Reuel Marc Gerecht of the United States, that believes the real promise of democracy lies with devout Muslims. In Feldman's first book, ''After Jihad,'' published just before he left for Iraq, he argued that the desire for democracy is widespread among Muslim believers, much more than the desire for violent jihad, and that Islamists should therefore be given a chance to rule.

Scholars don't often get to test their theories in the field. Feldman did in Iraq. As a constitutional adviser, Feldman helped shape Iraq's Transitional Administrative Law, the interim constitution and political road map for the country's transition from occupied territory to sovereign, democratic nation. ''What We Owe Iraq: War and the Ethics of Nation Building'' is a product of that experience. The book, like its author, is an unusual blend: part theoretical treatise, part political analysis, part memoir. Above all, it is a plea to the American conscience to take seriously the responsibility the United States has assumed to help the Iraqi people build the democracy Feldman believes they need and deserve.

When the United States invaded Iraq, Feldman argues, it did more than topple a tyrant. It undertook a ''trusteeship'' on behalf of the Iraqi people. Aware of ''the legacy of paternalism . . . inherited from the ideology of empire,'' Feldman argues that the nation-building task can be ''salvaged ethically only if it is stripped down to the modest proposition that the nation builder exercises temporary political authority as trustee on behalf of the people being governed.'' Readers unfamiliar with the style of academic discourse Feldman often employs in this book may wish to remain so. But the core of his thesis is powerful and important.

And it's not only the Iraqis who have an interest in Iraqi democracy, Feldman says. The United States and Europe have for too long erred both morally and strategically in supporting authoritarian governments in the Arab world. In Egypt, Saudi Arabia and elsewhere, Islamist terrorists ''have long been motivated by their grievances against the authoritarian states in which they live.'' Feldman points out that it was a ''cadre of Egyptian Islamist terrorists, defeated and thus displaced from their traditional battle against the Egyptian state in the 1990's,'' who ''joined forces with Osama bin Laden to create Al Qaeda.'' The answer to the threat of Islamic terrorism, he says, is to engage in nation-building ''aimed at creating democratically legitimate states that would treat their citizens with dignity and respect.''

While many argue that the Iraqis are not ready for democracy, Feldman insists it is the only system that can work. Without exaggerating what elections can accomplish, he makes a practical point often overlooked by skeptics. The diverse complexion of Iraqi society, he observes, means that no single group has the power to impose peace and stability. In order to succeed, an Iraqi government must be accepted as roughly legitimate by a broad cross section of Shiites, Kurds and Sunnis. But how can American officials or any outsiders, or even any Iraqi, know what the people will consider legitimate without asking them? Democracy, Feldman writes, is ''not merely the best political arrangement,'' it is ''the only option other than chaos.'' It helps that Iraq's Grand Ayatollah Ali al-Sistani appears to be the kind of Muslim leader Feldman is counting on: a Shiite cleric who by word and deed has so far proven himself sincerely committed to democracy. One gets the sense that Feldman and Sistani were tacit allies in pushing for an Iraqi state that can be both Islamic and free.

Feldman writes little about his own work in Iraq, and readers will be left wanting more. Probably he did not wish to violate the confidences of the Americans and Iraqis with whom he worked. He is the un-Richard Clarke, a public servant with an ethical compass. Still, there is no mistaking his disappointment with some of the Bush administration's failures in Iraq.

The most tragic was the failure in the early days after the invasion to fulfill the ''first duty'' of an occupying power: providing basic security. Much has been made of the looting that occurred immediately after the fall of Saddam Hussein's regime, but Feldman notes the essential point: by allowing the looting to proceed, American forces sent a clear message ''that the United States was not in charge, and that no one else was, either.'' Iraqis had to seek security for themselves in what was for a time a state of anarchy, and it was hardly surprising that they turned to their own kind for protection. Feldman says that it was not ''ancient'' ethnic and religious differences that empowered armed militias, but the human instinct for survival. ''Had there been half a million U.S. troops on the ground,'' he insists, ''it is highly likely that there would have been little looting, no comparable sense of insecurity and therefore a reduced need for denominational identities to become as dominant as they quickly did.''

The United States failed the Iraqi people again, he writes, when, in the winter and spring of 2004, it did not take the necessary steps to put down the growing insurgency. Although Feldman does not say so, much of the blame for this moral and strategic failure must fall on Secretary of Defense Donald Rumsfeld, whose responsibility it was to have enough forces on the ground -- not only for war fighting but also for the nation-building that followed. America's efforts in Iraq have never fully recovered from this monumental error.

Despite this and other American mistakes, however, Feldman believes success is still possible -- but only if the American people understand and make good on the moral obligation they have incurred. He worries that the next administration will be looking for exits, and that ''no one is asking what obligations we might have to the Iraqis whose government we deposed and whose country we occupied.'' And by ''we,'' Feldman writes, he really means ''you, the reader . . . no matter your views on war and reconstruction in Iraq or elsewhere.'' As American citizens, Feldman insists, we are all responsible for what happens in Iraq.

Robert Kagan is senior associate at the Carnegie Endowment for International Peace and author of ''Of Paradise and Power: America and Europe in the New World Order.''