In the week since the attacks on Paris, there has been a great deal of talk about waging war on the Islamic State, but scant clarity about how such a war might succeed. In a season when the improvisations of Vladimir Putin shape geopolitics, and those of Donald Trump shape American politics (Trump has remarked that Putin is “getting an A” for leadership), it is perhaps unsurprising that public discourse about what comes next has been informed by opportunism and incoherence. Yet even the sober, often stirring rhetoric of the French President, François Hollande, has often elided the main problem, which involves aligning aims with realistic means. “France is at war,” Hollande told his parliament last week, as French jets struck Raqqa, Syria, the Islamic State’s self-declared capital. He vowed to “eradicate” the organization. But how, and how long will it take?

In 2004, James D. Fearon, a political scientist at Stanford, published a study, “Why Do Some Civil Wars Last So Much Longer Than Others?,” in which he and a colleague analyzed scores of civil wars fought between 1945 and 1999. Some of the findings were intuitive: civil wars end quickly when one side has a decisive military advantage over the other; poor countries with natural resources to export often have long internal wars, because whoever controls the resources also controls the national treasury. Other findings were novel, such as the fact that wars following coups d’état tend to be short. In another study, “Ethnicity, Insurgency, and Civil War,” Fearon and the political scientist David D. Laitin discovered that even though in nations with exceptional ethnic pluralism, like Syria and Iraq, lines of conflict may be defined by ethnic identity, pluralism itself is not a notable predictor of civil war; poverty is a much more significant factor.

Rereading these works in light of the infuriating problem of the Islamic State, two discouraging findings stand out. In 1945, many civil wars were concluded after about two years. By 1999, they lasted, on average, about sixteen years. And conflicts in which a guerrilla group could finance itself—by selling contraband drug crops, or by smuggling oil—might go on for thirty or forty years. The Revolutionary Armed Forces of Colombia, or farc, has been around since 1964, sustained in no small part by American cocaine consumption.

The Islamic State is an oil-funded descendant of Al Qaeda in Iraq, a branch of the original Al Qaeda, which was formed in 1988. According to the C.I.A., isis has at least twenty thousand armed fighters; some estimates put the number much higher. It controls large swaths of territory, including major cities, such as Mosul. It is unusually barbarous, and good at Twitter. Its millenarian ideology of hatred and extermination poses a threat across borders. Yet its army and its sanctuary, in Iraq and Syria, are not, in a structural sense, exceptional.

From the American intervention in Somalia, in 1992, through the French intervention in Mali, in 2013, industrialized countries have been able to deploy ground forces to take guerrilla-held territory in about sixty days or less. The problem is that if they don’t then leave, to be replaced by more locally credible yet militarily able forces, they invite frustration, and risk unsustainable casualties and political if not military defeat. This has been true even when the guerrilla forces were weak: the Taliban possesses neither planes nor significant anti-aircraft missiles, yet it has fought the United States to a stalemate, and the advantage is now shifting in its favor.

If President Obama ordered the Marines into urgent action, they could be waving flags of liberation in Raqqa by New Year’s. But, after taking the region, killing scores of isis commanders as well as Syrian civilians, and flushing surviving fighters and international recruits into the broken, ungoverned cities of Syria and Iraq’s Sunni heartland, then what? Without political coöperation from Bashar al-Assad, Russia, Iran, Hezbollah, Iraqi Shiite militias, Turkey, the Al Qaeda ally Al Nusra, Saudi Arabia, the Gulf States, and others, the Marines (and the French or nato allies that might assist them) would soon become targets for a mind-bogglingly diverse array of opponents.

Syrian rebels overwhelmingly regard Assad’s regime as their main enemy, and for good reason: his forces have killed more Syrians than anyone else has. In the absence of a political agreement with Assad or his removal from office, it is impossible to conceive of a Muslim-majority occupation force that would be able and willing to keep the peace after the Marines departed. Some may argue that it would be worthwhile, nonetheless, to wipe out the Islamic State on the ground and deal with the fallout later. After Paris, such an approach may hold emotional appeal. After Afghanistan and Iraq, however, it is not a responsible course of action.

Analyses like James Fearon’s suggest that there are perhaps two ways to end, or at least to contain, long wars. One is to accept that success will be a long time coming, and to adopt a posture of military and diplomatic patience and persistence. That may yet lead to the farc’s disarmament. The other is to negotiate aggressively to form international alliances, which will allow for a rapid, decisive use of force on the ground. The European Union activated a mutual-defense compact after the Paris attacks; nato could broaden the alliance by invoking Article 5 of its treaty, as it did after 9/11. Such coalitions can be swiftly effective. When Saddam Hussein invaded Kuwait, George H. W. Bush and James Baker pulled together an unexpected military alliance to force his retreat. In Afghanistan, George W. Bush overthrew the Taliban with worldwide support. Both actions eliminated the immediate threat, but neither resolved the targeted country’s underlying instability, or assured durable international security. (On Friday, Islamist terrorists staged a murderous raid on a hotel in Mali’s capital, Bamako, almost three years after the French-led intervention in that country.)

Barack Obama has all but ruled out a ground intervention in Syria or Iraq. Instead, last week he promised “an intensification” of the strategy he is already pursuing: Special Forces raids, air strikes, and diplomatic conferences to try to resolve the Syrian war, perhaps by declaring ceasefires or insuring Putin’s coöperation. “A political solution is the only way to end the war in Syria and unite the Syrian people and the world” against the Islamic State, the President said. Unfortunately, right now that looks no more realistic than a prolonged American occupation of Raqqa. Obama’s caution in the Middle East since the Arab Spring is a reminder that there are perhaps as many risks attendant upon inaction as upon action. The dilemmas suggested by Fearon’s research won’t evaporate; they will be on the desk of Obama’s successor. ♦