There is still time for the United States to control the growing sectarian threat in Syria and other countries, but the volatile cocktail of religious antagonism, national interests, and oil requires immediate and vigorous action.

As the Assad regime hurtles toward deserved collapse in Syria, I often think back to a warning I received from a friend 18 months ago. I was serving then as the U.S. ambassador in Baghdad and was focused on Iraqi problems. But my confidant, an Iraqi Kurd with a strong commitment to a unified, multi-sectarian Iraq, and who was no friend of Syrian President Bashar al-Assad, was worried about the uprising brewing in neighboring Syria. Unless the United States was able to influence events, he cautioned, a revolt might violently split Syria, and then Iraq and finally the region along sectarian lines.

The sense that Assad’s days are numbered has prompted worries that militant Sunni extremists might claw their way to the top in Damascus. A greater and related danger, however, is that the uprising will degenerate into a Sunni-Shiite conflict that could spread beyond Syria’s borders and further destabilize the Middle East.

Already, reports are mounting that sectarian violence is commonplace in Syria and beginning to take hold in neighboring Lebanon. The Iranians and Assad have done their part to aggravate the problem by stoking fears among Iraqi Shiites and other Shiite groups about the consequences of a Sunni triumph in Syria.

But even without Iranian meddling, the danger is urgent. For Iraqi Shiites, the birth of the Shiite-Sunni split is as vivid as if it had happened yesterday. In fact, the divide began in Karbala more than 13 centuries ago, when forces from what is now Syria, who became Islam’s Sunni branch, put to the sword the prophet Mohammed’s grandson Hussein and many members of what became the Shiite sect. Fueling Shiite anxieties today is the fact that al-Qaeda in Iraq has taken its anti-Shiite violence to Syria, and Arab leaders in the Persian Gulf have been reluctant to distance themselves from inflammatory Sunni clerics who have cast Arab Shiites as “apostates.”

Adding to this combustible mix, Hamas has left Syria, opting for solidarity with their fellow Sunnis instead of maintaining Iranian support, and Turkey has lined up with the conservative Arab states against Syria and Iran.

Assad’s fall would deprive Iran of an ally in a strategically vital region and could open the door to a representative, humane Syrian state. But such gains may never materialize if we do not handle the sectarian fallout of the Syrian uprising. Past conflicts in the Middle East have generally been waged by states, a phenomenon that has limited their geographic spread and allowed international responses using traditional diplomatic and military tools. But serious antagonisms between religious groups could easily burst into all-encompassing violence region-wide, undermining the “Westphalian” model of statecraft by shifting loyalty from governments and the rule of law to one’s religious brethren and a “hate thy neighbor” theology. If the sectarian fires grow, the first victims could be Lebanon and Iraq, with their unhappy history of Shiite-Sunni violence; the international community has barely managed to contain this in wars past. The upheavals in the Balkans in the 1990s — a violent fissure between Orthodox Christians and Muslims that eventually involved most of former Yugoslavia — provide a taste of what might be in store for the Middle East. Only concerted and forceful U.S. action averted a broader regional war in Europe in the ’90s, and the Middle East is an even tougher neighborhood. U.S. officials are not blind to this danger. They have been working with Iraqi political leaders to maintain a relative peace between Sunnis and Shiites and to encourage Iraqis to stay out of the Syrian crisis. If these efforts fall short, however, sectarian violence would be extremely difficult to contain.

As formidable as the problems are, additional steps could be taken. One would be to speed the end of the Assad regime and quickly restore order in Syria. After 18 months, the continued chaos merely fuels sectarian hatreds. President Obama’s recent warning to the Assad regime about the use of chemical weapons was important, but we need to consider limited military assistance to the Syrian resistance even without the backing of the U.N. Security Council.

Beyond that, we must maintain our values, even when reaching for the undeniable prize of an Iranian defeat in Syria. As in the Balkans, the United States needs to ensure that we vocally defend the sanctity of borders, the human rights of all groups and the responsibility of all governments to protect all of their citizens. We also need to underscore the international political and legal consequences for those who ignore these responsibilities. (Judicial actions, including against some former U.S. allies, are still underway in the Balkans for just these sorts of transgressions.)

Toward this end, we need to make clearer that, in contrast with the views of some of our friends in the region, Shiites are not America’s enemy and that our problem is with the Iranian regime and Assad. We should start by taking a stronger position on Bahrain’s oppression of its Shiite majority.

Those who see difficulty in this approach must keep in mind that the states where Shiites are a majority — namely Iraq and Iran — probably have more than 300 billion barrels of oil reserves between them. That is almost two-thirds of the reserves of the Gulf Cooperation Council states, including Saudi Arabia, and more than 20 percent of global reserves.

There is still time for the United States to control this growing sectarian threat, but the volatile cocktail of religious antagonism, national interests and oil requires immediate and vigorous action.

James Jeffrey is a visiting fellow at The Washington Institute and former U.S. ambassador to Iraq and Turkey.