Arm the Syrian Rebels. Now.

Until recently, among President Barack Obama’s most senior advisors on national security, an ironclad consensus reigned: Arm the Syrian rebels. In testimony before the Senate Armed Services Committee on Feb. 7, Defense Secretary Leon Panetta and Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, affirmed that they both supported the call by former Secretary of State Hillary Clinton and David Petraeus, former director of the CIA, to provide lethal support to the Syrian opposition.

What were the arguments that convinced Obama to overrule his advisors? We may never know, but one thing is clear: They were not based on a sober reading of the situation on the ground in Syria, where U.S. policy is caught in a contradiction between word and deed. Though the president has repeatedly called for Syrian leader Bashar al-Assad’s ouster, he has proposed no credible plan for achieving that goal.

Last week, for instance, Obama stressed that the United States had "joined with nations around the world in calling for an end to the Assad regime." No sooner had he made this statement, however, than he dispatched Vice President Joseph Biden to attempt — once again — to engage Russia on a solution to the conflict. But reliance on mediation from Moscow — with its emphasis on an Assad-led transition — has proved to be fundamentally flawed. Assad will never preside over his own removal.

The diplomatic back-and-forth has come at the expense of decisive steps toward regime change. Obama has been right, after a decade of war, to ask hard questions about whether greater U.S. involvement can really work in the interests of either Syria or the United States. But his hands-off policy has now proved to be self-defeating. In the absence of American assistance, the rebels’ momentum has stalled, and the battles for Damascus, Aleppo, and Syria’s other strategic centers have devolved into a grim stalemate.

Meanwhile, Syrian society is fragmenting, and sectarianism is on the rise. While Jabhat al-Nusra, the local al Qaeda affiliate, is growing ever stronger, the Iranians and Hezbollah have doubled down on their support for the regime. Both have, for example, sent forces to fight alongside the Syrian army. In addition, they are training and equipping the Jaysh al-Shabi, a Syrian government-controlled force that, according to at least one Iranian source, is modeled on the Basij militia of the Islamic Republic. Iran is also providing economic aid and propaganda support.

The polarizing influence of Iran and al Qaeda portends a further escalation of sectarian violence, which will inevitably spill over into surrounding countries. To prevent the worst, the United States must assume a greater leadership role, which, as the president’s advisors have made clear, means building the capacity of the Free Syrian Army (FSA), a network of nationalist and secular-leaning rebel brigades. This would not necessarily require direct and sustained American military intervention, but it would entail arming the FSA and helping it to develop a countrywide military strategy.

From a purely military point of view, the rebels need help neutralizing the weapons that give the Syrian state its greatest advantages — namely, armor and fixed-wing aircraft. The provision of light anti-tank weapons would go a long way toward stopping Assad’s tanks. However, eliminating the regime’s air superiority, which rebels and civilians fear the most, is a thornier challenge. Here the United States and the international community have a crucial role to play in projecting a credible threat of force to stop Assad from indiscriminate bombing. While it may not be necessary to impose a Libya-style no-fly zone (NFZ), it is imperative to keep the threat on the table and to be willing, if required, to carry it out. An obvious alternative to an NFZ is to provide man-portable air-defense systems (MANPADs). But the legal and prudential restrictions are considerable. The use of these systems would require a stronger partnership between the FSA and key regional allies than currently exists.

In addition to weaponry, the FSA needs training, resources, and intelligence support. It currently lacks a sound military strategy. Only the Americans, working together with Arab partner nations, have the requisite diplomatic and military resources to help the FSA develop this capacity. It is often said that the United States has no successful track record of providing this kind of assistance. But that is simply false. In fact, in recent months it has enjoyed a number of quiet successes in Yemen. With a very light footprint, the Pentagon has helped train and equip the Yemeni army, giving it the wherewithal to retake territory that it previously ceded to al Qaeda. While the American drone campaign has grabbed the headlines, the effort to build partnership capacity holds out the greatest long-term promise. The partnership being developed in Yemen is precisely the model that is needed in Syria.

It is important to remember that arming the FSA is a political act. The most important decision of all is simply to provide lethal assistance. The goal of the operation is to build a force on the ground that is more likely to respect American interests and that is committed to building a nonsectarian, stable Syria. Even the provision of light weaponry would be a good start to this project.

This policy does entail the risk of unintended consequences. Some arms may flow to al Qaeda. Some groups may take American aid and then turn against the United States. But inaction also carries risks. The current hands-off policy has hardly succeeded in preventing extremists from acquiring arms. It has simply given them time and incentive to develop their own independent sources of external support.

By establishing itself as the most important international player shaping the conflict inside Syria, the United States will lay the groundwork for helping the Syrian people forge a genuine national dialogue on the nature of their transition. This should include the creation of a national platform that brings together Syria’s diverse ethnic and religious communities — including Sunnis, Shiites, Alawis, Christians, and Kurds, as well as tribal and religious figures — to discuss the future of the country. In particular, it should include Alawis who enjoy wide legitimacy within their community, but who are also willing to talk about a post-Assad Syrian regime.

At the same time, the United States should bring together key international and regional powers to create an international steering group. This group — including China, Russia, Turkey, and key Arab and European states — should agree on a number of basic goals for the transition and set benchmarks for their effective implementation. The immediate focus should be on protecting civilians, minorities, and vulnerable groups through the creation of an international stabilization force; addressing humanitarian issues; safeguarding chemical and other unauthorized weapons; and supporting Syrian-led transitional governance and transitional justice efforts.

For this to succeed, Obama must first persuade Russia to abandon its demand that Assad play a role in the transition. If Moscow remains defiant, however, the president must be willing to pursue an independent policy — while still keeping the door open for Russian President Vladimir Putin to eventually join the international consensus.

The Syria challenge is difficult. Its intractability is what initially made nonintervention attractive. But developments on the ground have since made it an increasingly dangerous option for American interests. It’s time Obama listened to his foreign-policy and national security advisors.