The ongoing clashes between rival rebel factions will likely be protracted and indecisive, and the resultant diversion of effort is already working to the regime's advantage.

The outbreak of serious fighting earlier this month between jihadist militants and a loose alliance of more moderate rebel factions represents a potentially critical stage in the Syrian conflict. This war-within-a-war reflects all the complexity of the wider rebel struggle against the Assad regime: many different groups engaged, murky and shifting relationships among the players, confusing battles with ambiguous outcomes, and the jumbling of forces on the ground. At the same time, fighting continues between rebel and regime forces, sometimes on virtually the same ground simultaneously. The inter-rebel conflict is the war's most important military development since Hezbollah's direct intervention in spring 2013. How and when it will end is unclear -- at present, it appears to be protracted, costly, and of most benefit to the regime. Yet the United States can still influence the outcome favorably by aiding the moderate elements fighting the extremists.

WHO IS FIGHTING, AND WHY?

On January 3, serious hostilities broke out between al-Qaeda-aligned jihadist group the Islamic State of Iraq and al-Sham (ISIS) and an ad hoc alliance of more moderate factions. The latter alliance includes fighters from the Islamic Front (IF), a rebel umbrella organization formed late last year, as well as Jaish al-Mujahedin and the Syrian Revolutionaries Front, another umbrella group. In one of many wrinkles, at least some forces from the jihadist group Jabhat al-Nusra -- a U.S.-designated terrorist entity -- have fought on the IF's side as well.

For the more moderate Islamist factions and their secular allies, the clashes represent the first serious effort to curb ISIS's power and excesses in the north, and a chance to return the revolution to a more moderate course. For ISIS and its supporters, the fighting is an opportunity to inflict a telling defeat on their armed rivals in the opposition. And for the regime, it is an opening to exploit -- a chance to inflict more losses on rebel forces and regain more lost territory.

The proximate cause of the fighting was a series of specific actions by ISIS that were deemed unacceptable by the IF and its allies; for example, the organization had killed commanders and officials from rival groups and fired on anti-ISIS demonstrators. These acts occurred against a backdrop of increasing ISIS efforts to impose strict Islamic codes in areas under its control. Thus, the fighting is also about the struggle to define the revolution along extreme or more moderate Islamic lines: ISIS wants to extend its jihadist vision across the rebellion, while its opponents hope to curb the group's power and excesses.

NATURE OF THE CLASHES

Most of the fighting has occurred across a broad area of rebel-controlled northern and eastern Syria, including the provinces of Idlib, Aleppo, and Raqqa, with small localized outbreaks in provinces such as Hama and Latakia. It lacks clear fronts, with forces thoroughly mixed on the ground and clashes occurring at many locations. In some areas (e.g., Qalamoun and Hasaka), the combatant rebel groups continue to cooperate in actions against regime forces even as they fight against each other, depending on local conditions, networks of affinity and allegiance, and personal relationships among units and commanders.

The combat itself seems to consist mostly of small-unit actions -- tens of men rather than hundreds -- fought for control of border crossings, key towns and villages, unit bases and headquarters, and arms-storage locations. Actions reported thus far include ambushes, checkpoint attacks, cutting lines of communications, sieges, and assaults on positions and facilities. Both sides are using heavy weapons captured from the regime, including tanks, mortars, and antiaircraft weapons. ISIS has also used suicide attacks and car bombings to strike its opponents, even reportedly forming special suicide units. One map posted on opposition websites on January 18 showed almost fifty locations of reported ISIS suicide attacks against civilians and rival rebels.

In some cases the clashes have been heavy, producing numerous casualties. The Syrian Observatory for Human Rights has reported 426 killed in action for ISIS and 760 for the IF and its allies. Many more have no doubt been wounded, and both sides have reportedly executed some prisoners as well.

Meanwhile, both sides are engaging in information operations to undermine the allegiance of rival units and appeal for support from the population. Each faction has had some success in this regard, generating defections, declarations of support, and public rallies in their favor. Adding to the complexity of the situation, the combatants have also negotiated local ceasefires in some areas, as well as prisoner exchanges and safe passage of personnel through enemy lines.

RESULTS

No clear victor has emerged from the fighting. Initially, the IF and its allies made significant gains against widely dispersed and apparently unprepared ISIS forces. This demonstrated that ISIS could be defeated by determined action. Since then, ISIS has recovered at least somewhat, mobilizing and redeploying its forces and preparing for a long fight. In addition to initiating a suicide bombing campaign, the group seized full control of Raqqa city and other key areas of Raqqa province, considered its center of power. As the fighting has evolved, the IF and allies have appeared to be stronger in Idlib province and to a lesser degree in Aleppo, while ISIS is strongest in Raqqa and able to draw on further support from Deir al-Zour. Maps posted on opposition websites show a very confused situation on the ground, with disputed locations changing hands daily.

Uncertain outcomes aside, the clashes have been costly for the opposition. All of the casualties are rebels whose previous main business was fighting the regime -- in ISIS's case, at least nominally. The rebels are also expending ammunition and weapons, resources they have never had in abundance. Every rebel casualty and every bullet fired in this secondary conflict directly benefits the regime. The fighting is creating its own refugee flow as well, displacing the already displaced.

In short, the inter-rebel fighting represents a serious diversion of effort, disrupting operations against the regime and reducing pressure on regime positions in areas such as Raqqa. ISIS has reportedly pulled out of some defensive positions in order to concentrate on its rivals, and the internecine clashes have apparently allowed regime forces to make gains in Aleppo. There are also many reports and rumors that ISIS and regime forces are explicitly or tacitly cooperating. If so, this is further to the regime's benefit, both operationally and in terms of sowing suspicion among rebel forces.

IMPLICATIONS AND OUTLOOK

Much will depend on the outcome of the fighting and how quickly it comes. More than three weeks into the clashes, no decisive result appears imminent: ISIS is dominant in some areas, the IF and its allies in others. A decisive win by either side would do much to determine the rebellion's future course, but a protracted and inconclusive fight -- as seems more likely -- would work to the regime's long-term advantage and support its narrative of the war. Damascus has every incentive to keep this conflict going and will likely do what it can to make sure it does.

Although total victory by either side is unlikely, an eventual win by the IF and its allies would be a boon to the moderate Islamist and secular opposition. It would also relieve some of the pressure imposed on the civilian population by ISIS violence. If ISIS prevails, however, outside support for the rebellion would wane even further, and the plight of civilians in rebel-held areas would become still more difficult.

In this situation, aiding the more moderate rebels -- potentially including some Islamists -- in their fight against ISIS makes sense. Such assistance could lead to the one outcome that is in the interests of the United States and its allies: a moderate rebel win against the extremists, and perhaps ultimately against the regime.

Jeffrey White is a Defense Fellow at The Washington Institute and a former senior defense intelligence officer.