THE DEFECTION of Syria’s prime minister to Jordan on Monday prompted yet another White House declaration that the regime of Bashar al-Assad is “crumbling.” While we hope that this is the case, it seems more likely that the administration’s prediction will prove as premature as its previous announcements of Mr. Assad’s imminent downfall, dating back a year. The grim reality is that the regime and the brutal war it is waging in Syria’s cities is likely to go on and on — unless the United States abandons its policy of passivity.

Experts on Syria say that Mr. Assad’s power structure is not so much crumbling as fragmenting along ethnic lines. What remains is a hard core of military units and their leaders, drawn from the minority Alawite sect. The defecting prime minister, like almost all of the 40 other senior government and military officials who have switched sides, is from the majority Sunni population. Other ethnic groups in Syria are going their own ways: The Kurds in the northeast of the country have taken over their own territory and hope to establish an autonomous region like that in Iraq.

What remains of the regime is still able to mount formidable military operations, like the current assault on opposition-held districts in the city of Aleppo. Its scorched-earth strategy leaves no room for compromise. As a new report from the International Crisis Group grimly notes that “the regime has essentially been stripped down to a broadly cohesive, hardcore faction fighting an increasingly bitter, fierce and naked struggle for collective survival. It is mutating in ways that make it impervious to political and military setbacks, indifferent to pressure and unable to negotiate.”

The report also notes that opposition forces had become increasingly anti-Alawite and that fundamentalist Islamic ideology was on the rise, as was a “loss of faith in the West.” Other sources report that hundreds of fighters from al-Qaeda have infiltrated the country and are steadily raising their profile.

All this underlines a point made five months ago by some of the State Department’s own Syria experts: The longer the fighting in the country goes on, the more it evolves toward open sectarian war, promotes extremist ideology and undermines the possibility of an eventual settlement based on pluralism and democratic principles. That’s why the Obama administration was foolish to waste the intervening months backing a feckless U.N. diplomatic initiative and why its current attempts to promote a “managed transition” from the Assad regime are equally misguided.

The only workable policy in Syria is one that aims at ending the civil war as quickly as possible with a victory for the opposition. A coup by regime elements that removes Mr. Assad may still be possible, but only if generals perceive that the war is lost. That means supplying the rebels with the arms they need to stop the tanks and planes of the Assad forces. To protect civilians, safe zones can be established along Syria’s borders with Turkey and Jordan, with help from Turkey or NATO.

Taking such steps would help the United States establish relations and exert influence over those forces that will likely be the next leaders of Syria — the commanders of the Free Syrian Army. A major effort should be made to persuade them to plan for a postwar order in which Alawite and other minorities are protected and a transition to democracy is organized with international assistance. Though U.S. diplomats have been pushing that agenda, they have focused mainly on exiled opposition leaders, rather than those doing the fighting. As veteran diplomat James Dobbins, who helped guide U.S. interventions in the Balkans and Afghanistan, noted in congressional testimony last week, “American influence and ability to advance such goals will tend to be in direct proportion to the help the United States provides the opposition in their fight to overthrow the regime.”

By refusing to step in, the Obama administration is merely ensuring that Syria’s future leaders will be more resistant to the West and perhaps more open to groups such as al-Qaeda. It is also giving the enduring hard core of the Assad regime the space and the opportunity to fight on.