Anthony H. Cordesman holds the Arleigh A. Burke chair in strategy at the Center for Strategic and International Studies.

Americans cannot afford to forget that they face more than one crisis in the Middle East. Critical as Egypt is, the situation in Syria continues to spiral out of control, affecting the security of Lebanon, Turkey, Iran and Iraq and giving Iran new opportunities.

The Assad regime continues to make gains and has less and less reason to negotiate. For all the talk of U.S. arms transfers, the Syrian rebels have problems moving arms and supplies across the Lebanese, Turkish and Iraq borders. Meanwhile, Lebanon and Iran are supplying the regime with volunteers and arms. The lack of outside support weakens the moderates among the rebels, while the upheavals in Egypt polarize Syria’s Sunnis and tend to empower the more extreme factions.

Even “success,” or the fall of Bashar al-Assad’s government, would result in a new government whose structure is unpredictable and that will inherit enduring political problems and regional tensions. On balance, however, there are clear humanitarian and selfish reasons for the United States to intervene.

At least 93,000 people have been killed — and probably close to twice that many — and up to 400,000 seriously wounded. The State Department estimated in May that 6.8 million Syrians need serious assistance, with 4.2 million internally displaced and 1.4 million outside Syria: That’s more than a third of the country’s 22.5 million people. Economic and social costs will rise indefinitely unless the Syrian people get a government the world can accept.

This may not be enough to sway U.S. public opinion, or congressional action, at a time when Americans are war-weary and facing a federal budget crisis and competing strategic demands. But although Washington cannot guarantee an outcome in Syria simply by arming and supporting the rebels, doing nothing could create a much broader threat to U.S. interests and our allies in the region.

What started as a civil conflict more than two years ago now threatens to fuel a major conflict between Sunnis and Shiites throughout the Muslim world. The conflict is dividing Lebanon and giving Hezbollah and other extremists a larger foothold there. It is also creating problems in Jordan and Turkey, pushing Iraq toward civil war and making Iraq’s Shiite leadership more dependent on Iran.

If Assad succeeds in crushing the opposition or otherwise maintains control over most of Syria, Iran will have a massive new degree of influence over Iraq, Syria and Lebanon in a polarized Middle East divided between Sunni and Shiite. Minorities will be steadily driven into exile. This would present serious risks for Israel, weaken Jordan and Turkey and, most important, give Iran far more influence in the Persian Gulf, an area home to 48 percent of the world’s proven oil reserves.

If Washington arms the rebels and they still lose, the United States will at least have shown its willingness to make decisions and honor its commitments. It will have shown it will make good on its words and support its allies.

More advance transfers of U.S. arms, such as the antitank guided missiles and surface-to-air missiles that the rebel commanders say can shift the balance, could also be supplied and funded by our gulf allies. They do not have to be cutting-edge U.S. systems, and the rebels already have some Chinese and Russian man-portable surface-to-air missiles, as well as systems that could be a major threat to civilian targets, should they fall into extremist hands. It is unlikely the United States can control such transfers from friendly Arab gulf states if we do not supply the rebels, and it is far more likely that we can have a major influence on which faction gets such arms if we work with the rebels — particularly now that Qatar seems more willing to cooperate with the United States and Saudi Arabia.

The costliest and riskiest U.S. option is direct intervention. To be truly effective, this would require a “no-fly zone” over all of Syria, covering all air and helicopter movement. The United States could, however, begin with arms transfers that would have a far greater chance of success if they included man-portable surface-to-air missiles and antitank guided weapons. U.S. officials could make clear that either the rebels will succeed with such weapons, leading to a negotiated departure of Assad’s government and the installation of a new national government, or the United States will join with allies in creating a no-fly zone.

No one is advocating a serious U.S. air campaign, with substantial money committed and probably significant U.S. air casualties. But the United States should show its willingness to act if its allies join and can help defer the cost. Doing so would give the rebels enough of an advantage to force efforts to negotiate — and may well intimidate Assad’s forces into halting air operations without requiring a massive attack on Syria’s air bases. It would show that the United States is serious about strategic partnerships. It might help us persuade allies to back up their words with actions. And it might even show the Islamic world that there is an alternative to extremism and Sunni-Shiite conflict.