OPPONENTS OF U.S. intervention in Syria are adept at citing the risks of a more aggressive U.S. effort to bring down the regime of Bashar al-Assad. Weapons given to rebel fighters might end up in the hands of extremists, the skeptics say. U.S. air attacks or the creation of a no-fly zone would be challenged by formidable air defenses. U.S. intervention might increase the risk that the regime would resort to chemical weapons.

Above all, say the anti-interventionists, direct or even indirect U.S. engagement in the fighting would make Syria an American problem, saddling a war-weary country with another difficult, expensive and possibly unworkable nation-building mission.

These are serious objections, though we believe that some of the risks, such as the spread of weapons to jihadists, can be mitigated, while others, such as the strength of Syrian air defenses, have been exaggerated. Our greater concern is about the side of the discussion critics of intervention usually leave out — which is the risks that are incurred by failing to intervene.

What will unfold in Syria if the Obama administration persists with its policy of providing humanitarian and other non-lethal aid while standing back from the fighting? The most likely scenario is that Syria fractures along sectarian lines. An al-Qaeda affiliate, Jabhat al-Nusra, is already consolidating control over a swath of northeastern Syria; remnants of the regime, backed by Shiite fighters from Lebanon’s Hezbollah movement, could take over a strip of the western coastline.

Such a splintering would almost certainly spread the sectarian warfare to Iraq and Lebanon, as it has to some extent already. That could cause the collapse of the Iraqi political system that was the legacy of the U.S. mission there. Chemical weapons stocks now controlled by the Assad regime would be up for grabs, probably forcing further interventions by Israel in order to prevent their acquisition by Hezbollah or al-Qaeda. Jordan, the most fragile U.S. ally in the Middle East, could collapse under the weight of Syrian refugees. Turkey and Saudi Arabia, which have been imploring the Obama administration to take steps to end the war, could conclude that the United States is no longer a reliable ally.

Of course, some of these consequences may come about whatever the United States does. But the best way of preventing them is to quickly tip the military balance against the Assad regime — something that would probably require an air campaign as well as arms for the moderate opposition. If the regime’s fighting strength is decisively broken it might still be possible to force out the Assads and negotiate a political transition, as Secretary of State John F. Kerry aspires to do. For now, with the regime convinced it is winning, there is no such chance — and with each passing month Syria’s breakup comes closer to reality.

In short, there are substantial risks for the United States if it intervenes in Syria but also grave dangers in its present policy. On Tuesday President Obama said his job was to “constantly measure” what actions were in the best U.S. interest. It’s not an easy calculus, to be sure. But for two years, as Mr. Obama has heeded the warnings about U.S. engagement, the situation in Syria has grown more dangerous to U.S. interests. There are no good options, as everyone likes to say. But it’s becoming increasingly clear that the greatest risk to the United States lies in failing to take decisive action to end the Assad regime.