When President George W. Bush announced his commitment to help build a democracy in a post-Saddam Hussein Iraq in a Washington speech last Wednesday, he stated a goal that will be difficult, although not impossible, to achieve.

A successful war to disarm Iraq would leave the United States in control of the country and therefore responsible for establishing a new government. Simply to install another autocratic regime, like those that govern virtually all the other Arab countries, would betray American values, run counter to the broad global political trend toward democracy of the last three decades, and risk putting in power a Hussein-like figure who, like his predecessor, would seek to acquire weapons of mass destruction.

Promoting democracy in postwar Iraq is therefore in the interests of both the United States and Iraq. Just how successful such a policy would be cannot, of course, be known in advance. But it is possible to identify the principal sources of optimism that Iraqi democracy is possible, as well as the main obstacles to achieving it. There are two of each.

Democracy requires democrats, and there are democrats - people who embrace the democratic ideas of self-government and individual rights - in Iraq. These ideas are universal; they have constituencies everywhere. True, there are few outspoken democrats in the Arab world, but that is because in almost every Arab country - and especially under the current Iraqi regime - democrats are suppressed and even persecuted. As has happened in many countries around the world, when it is safe to do so many in Iraq will express their support for the kind of government the citizens of Western countries enjoy.

It is unsafe to express support for democracy in Iraq today because of Saddam Hussein. He and his regime stand as the principal obstacle to democratic government. His removal from power, which a successful war to disarm Iraq would accomplish, is a necessary condition for bringing freedom to the Iraqi people. That is a second reason for optimism that the United States can help to establish a democratic Iraq.

But the overthrow of the present government, desirable though it is, will not, by itself, be sufficient to establish the first democracy in the Arab world.

One reason for this has to do with the definition of democracy. It involves free elections, so that the people can choose (and remove) those who exercise power over them. But democracy has a second, equally important component. It consists, as well, of institutions - political parties, for example, and above all the rule of law, with protection for the rights of individuals and minority groups, which in turn requires an independent judiciary. While elections can be conducted quickly, institutions take time to build: The democratic institutions of the West are the products of decades, sometimes centuries, of development.

Democratic institutions in Iraq will have to be constructed almost from scratch. If not centuries, they will surely require years to establish. This suggests that an American postwar military presence may also be required for years, to protect these fragile new institutions. But whether the American public would support a protracted stay in the country, and whether the people of Iraq would welcome or even tolerate such a presence, are open questions.

If the absence of the appropriate institutions is one barrier to democracy in Iraq, another potential obstacle is the ethnic and religious composition of the country. Iraq's borders were drawn by the British after World War I to encompass three different groups: Shia Muslims, who make up about 60 percent of the population; Kurds, comprising 20 percent; and Sunni Muslims, the group to which Hussein belongs, who make up most of the remaining 20 percent.

It is conceivable that, with Hussein's dictatorial rule gone, Iraq will follow the pattern of the former Yugoslavia in the early 1990s, with the different constituent groups refusing to coexist peacefully within a single country. If so, democracy will not be established in Iraq, just as it has proved impossible to establish it in another place where three distinct groups do not wish to live together - post-Communist Bosnia.

All of this means that, if the United States leads a coalition to disarm Iraq by force, the task of nurturing democracy in the country after the war is over will be both unavoidable and difficult. And while the prospects for political freedom in Iraq depend most heavily on the commitment to democratic values and institutions of the Iraqi people themselves, the chances for democracy there will also rest on the strength and durability of the American commitment to supporting it.

Michael Mandelbaum, author of "The Ideas That Conquered the World," is a senior fellow at the Council on Foreign Relations and professor of American foreign policy at The Johns Hopkins School of Advanced International Studies.