It is important to keep in mind that despite its considerable strengths, the Islamic State is neither an all-powerful nor an undefeated force. The strategic success the group has had in Iraq and Syria is also a weakness, since it must now stretch its resources to fight on various fronts against multiple enemies. Indeed, we have recently seen the Islamic State face setbacks and outright defeats in Iraq and Syria. For instance, the Kurdish militia known as the People's Protection Units has continued to repel Islamic State attacks in the Kobane pocket, and other rebel groups have either defeated (in eastern Ghouta) or driven out (in northern Homs) isolated and vulnerable Islamic State units that found themselves far from the organization's core in eastern Syria.
Ultimately, the primary existential threat to the group remains internal dissent, particularly from overrun Syrian tribes and fellow Sunni rebels in Iraq that oppose its draconian rule. For now, the Islamic State has managed to pacify this internal opposition, particularly by violently making an example of the Sheitaat tribe in Deir el-Zour that dared to oppose it. Yet opposition tribes and the wider Sunni populace now under the Islamic State’s rule remain a constant threat to the Islamic State, consuming a considerable portion of the group's attention and resources.
The Islamic State is also far from being the only capable armed group fighting against Syrian President Bashar al Assad's regime. It is important to note that the Islamic State currently rules little more than 10 percent of the pre-war population of Syria, and much of the expansive territory it holds is sparsely populated desert. Furthermore, the number of core Islamic State fighters in Syria is estimated to range between 10,000 to 20,000, far less than the approximately 100,000 fighters that belong to other Syrian rebel groups.
However, the other rebel groups continue to suffer from critical disadvantages, particularly in terms of their inherent divisions, which affect revenue and supply sources. Yet these groups continue to find ways to cooperate with each other, as was demonstrated Aug. 28 when five of the larger groups in the eastern Ghouta area near Damascus formed a joint military council. Without underestimating its significant tactical skill and capability, it is important to remember that the Islamic State’s recent victories have largely occurred against isolated loyalist forces far from the regime’s core, while other rebel groups continue to attract the vast majority of the regime’s attacks and offensives. Overall, as a single unified fighting formation, the Islamic State is by far the strongest rebel outfit in Syria, especially when its fanaticism and expertise are factored into the equation. Nevertheless, when compared to entire rebel coalitions such as the Islamic Front, the Islamic State is not significantly stronger.
As the United States carries out reconnaissance flights over Syria, continues to bomb Islamic State positions in Iraq and seeks to organize a regional and international response against the group, it is important to remember that, just as in Iraq, there are more than two competing armed groups in Syria. The fate of the Islamic State will affect not only the organization itself and the regime, but also the many other diverse rebel actors who continue to play a decisive role in the trajectory of the Syrian civil war. The plethora of combatants and the complexity of the conflict, which has now raged for more than three years, will have to be taken into account by the United States as it reviews its options in Syria.