A s the international community struggles with how to emancipate Libya from Moammar Gaddafi’s blood-soaked tyranny, a more troubling spirit of extremism lurks in the background of the Arab Spring .

Islamism is very much a relic of the 20th century, a discredited orthodoxy of limited appeal to the Arab masses. History has shown, however, that well-organized parties of circumscribed appeal can nevertheless assume greater influence by exploiting the disarray of transition periods and divisions within democratic camps. The problem is compounded by the temptation among many in the West to appeal to the “political” wing of militant organizations such as Hezbollah or to reach out to “moderate” elements of Islamist parties. The challenge for Washington today is not to cling to some kind of ecumenical spirit but to actively choose sides and fortify the political center against forces of intolerance.

Many in the West presume that once Islamist parties are integrated into the political order, the burdens of governance — compromise, coalition-building and constituency maintenance — will inevitably lead them to dispense with their ideological past. Such liberal conceits do a disservice to the Muslim Brotherhood and its many offspring, denigrating their commitment to their dogma.

The ideology of Islamists is predicated on the notion that religion is a comprehensive belief system that is both eternal and transnational. The moderation that these groups have exhibited in the past few decades in places such as Egypt was pragmatism born out of compulsion, not some kind of intellectual evolution. Relieved of the constraints of Arab police states, they are free to advance their illiberal, anti-Western agendas.

The plight of Islamist associations resembles the communist parties that did so much to derail Europe’s liberal age in the 1920s. Like the Islamists, the communists never commanded much popular support, but they used their parliamentary and paramilitary presence to undermine the prospects of fledging democracies; Germany and Italy are two examples. In due course, their devotion to the Soviet Union and their subordination of national interest to the cause of the global proletariat did much to facilitate the rise of fascism.

Islamist parties can be counted on to similarly menace an inexperienced democratic order. Their deputies are extremely likely to press discriminatory legislation; their religious leaders will stimulate passions against women’s rights groups and nongovernmental organizations; and their militias will threaten secular politicians and civil society leaders who do not conform to their template. Such agitations may not garner absolute power but could still provide an opportunity for national militaries to end the region’s democratic interlude in the name of stability and order.

The answer, then, is not to exclude Islamist parties from political participation. A genuine democratic system will have to include all contending voices, however ill-intentioned and radical they may be. Washington’s challenge is to make certain that the region’s political transition does not culminate in the assumption of power by another military clique. So the United States and its allies must strengthen the political center and the democratic regimes that are coming to power in the midst of economic crisis and without the benefits of mature institutions.

A massive package of economic assistance to countries such as Egypt and Tunisia would tether these nations to the United States and would allow Washington to be clear that extremism in any guise will cause cessation of support. Even in an age of budgetary constraints, Washington may be able to generate substantial sums by channeling military aid to civilian pursuits and by collaborating with the European Union, the World Bank and the International Monetary Fund. The United States may not be able to determine the outcome of the Mideast uprisings, but it can still impose conditions and offer incentives that diminish the appeal and potency of militant actors.

Beyond such measures, Washington has the moral obligation of political partiality. During the Cold War, it did not remain passive as the forces of democracy battled the communist parties that sought to exploit post-World War II dislocation to advance sinister designs. The United States actively and at times covertly aided noncommunist forces throughout Europe, ensuring the defeat of powerful communist parties in France and Italy. In the context of the Middle East today, this means standing with emerging secular parties and youth activists as they seek to reinvent the region’s politics and finally push the Middle East into the 21st century.

The notion that America’s interventions in the Arab world have made it a toxic agent that should stand aside is a presumption of the Western intelligentsia — and one rejected by Arab protesters, the majority of whom have not uttered anti-American slogans. The springtime of the Arab world offers the United States an opportunity to reclaim its values and redeem its interests. America has a stake in the future of the Middle East and should not shy away from cultivating the nascent democratic movements sweeping the region.

The writer is a senior fellow at the Council on Foreign Relations.