What, then, of the U.S. war against the Islamic State? ISIS is more self-reliant than many contestants in this conflict, funding its war effort mainly by taxing economic activity in areas it controls. The Islamic State lacks the kind of outside benefactors that fuel Assad’s or Jabhat al-Nusra’s or the Iraqi government’s war efforts. And as the group’s ongoing incompetent governance destroys the economy in areas it controls, ISIS will likely run out of money sooner than many of the war’s other combatants. Indeed, it may already be showing signs of this: In late November 2015, the group slashed its fighters’ salaries in half, reporting in Western media suggests its revenues have plummeted, and the group has reportedly tried to create cash through exchange-rate manipulations (a doomed strategy for small economies, as former finance ministers from Mexico, Thailand, Argentina, and other developing economies could attest).
The real problem, however, runs much deeper than just the Islamic State. The war in Iraq and Syria pits a host of groups against one another, many of whom are almost as dangerous to U.S. interests as the Islamic State, and many of whom seek the same status of jihadi vanguard against the West that ISIS now enjoys. ISIS itself achieved this position at the expense of the al-Qaeda-affiliated Jabhat al-Nusra, with whom it competes actively for recruits and resources. Even if some U.S.-allied proxy succeeds in conquering the nominal Islamic State capital of Raqqa and pulling down the ISIS flag from its citadel, this is unlikely to end the war, stabilize Syria, or even remove the threat of jihadi terrorism against the United States from Syrian (or Iraqi) soil—it would just open the war to its next phase, in which Islamic State’s rivals compete for the status that group had enjoyed before them. In the absence of a mutual willingness to end the war and accept some new model of representative governance, even great progress against ISIS does not realize U.S. interests in the conflict—which are to end the terrorism threat, the humanitarian crisis, and the danger to regional stability, not merely to occupy the city of Raqqa. A mutual willingness to end the war on the part of most or all of today’s warring factions, however, seems a long way off.
In this context, real U.S. leverage to bring about a real end to the war—and actual realization of American interests in that war—is distinctly limited. None of the proposals popular in today’s Washington debate offer any meaningful prospect of achieving this. According to U.S. military doctrine, to defeat even an insurgency (much less a proto-state like ISIS) and stabilize a threatened population requires something like 20 counterinsurgents for every thousand civilians. That means 50,000-100,000 well-trained troops would be needed to hold the area now under Islamic State control (depending on how much of the population has fled), much less the rest of Syria. No one is now proposing a realistic plan to accomplish anything close to this—whether such a force comprises American troops, Iraqis, Kurds, Saudis, Turks, or anyone else. In the absence of this, bombing raids or offensives from Iraqi or Kurdish allies can accelerate the rate at which ISIS burns through its capital and perhaps hasten the day when the Islamic State is replaced by the next militant group in the queue—but limited efforts of this kind cannot end the war.