FOR MONTHS the United States and its allies have grappled with how to respond to a mass movement of peaceful protests in Syria and the government’s despicably violent response to them. Too slowly, the Obama administration has moved from urging dictator Bashar al-Assad to implement reforms to imposing sanctions and calling for him to step down, while seemingly embracing a strategy of “leading from behind.” Now it appears the administration and other outside powers could soon be faced with a very different situation: war between Mr. Assad’s dwindling forces and a rebel army made up of military defectors and volunteers. That would require a stronger, quicker and more forward-leading U.S. response.

A number of news reports in the past week have cited diplomats and Syrian sources as saying that armed resistance to Mr. Assad’s assaults on the population has begun to appear — including in the central towns of Homs and Rastan (where heavy fighting was reported Tuesday) and near the Turkish and Iraqi borders. The New York Times quoted an unnamed U.S. official as estimating there had been 10,000 defections from the Army and security forces, and that several hundred of these had joined one of two rival movements — the Free Syrian Army and Free Officers Movement.

The appearance of such forces is not to be welcomed, even by those hoping for an end to the Assad regime. Violence will push extremists to the forefront, justify even more brutal repression by the government and possibly transform what has been a broad pro-democracy movement into a sectarian war. Fighting could spread to Syria’s neighbors, including Lebanon and Iraq, and invite intervention — covert or otherwise — by outside powers, beginning with Iran. But as a State Department spokesman pointed out Monday, the incipient rebel movements are an inevitable “act of self-preservation” against “a regime that continue[s] to use violence against innocent, peaceful demonstrators.”

The administration is right to hold Mr. Assad responsible for provoking civil war, but the question is what can be done about it. There are some obvious first steps, including urging the organized Syrian opposition, which recently formed a national council, to reject violence at an upcoming meeting in Istanbul. Syria’s neighbors should seek to choke off arms supplies to the regime — as Turkey is doing. Some fighting might be averted if safe zones for Syrians fleeing government persecution were established along the borders, either with Turkey or Iraq.

In the end, the only way to avert a Syrian civil war may be for Mr. Assad’s regime to collapse. Having ruled out armed intervention of its own, the outside world can’t force this outcome; but the United States could drop its back-seat approach and lead a more aggressive effort to raise the pressure on Mr. Assad. The administration can press Russia, China and the Arab League to endorse tougher sanctions, and urge Turkey to break with the regime and provide protection for refugees. It would be far easier for the United States to act energetically now than to deal with the crisis that a real civil war would create.