President Obama is practically allergic to “mission creep,” the slow but out-of-control expansion of a war’s aims, terrain, and scope. But he’s coming under intense pressure to abandon his constraints and succumb to escalation in the fight against the terrorist group known as ISIS.

So far he’s resisting, but given the nature of the fight it’s going to be hard.

Until this month, Obama’s anti-ISIS strategy shrewdly circumscribed America’s involvement. The focus would be on Iraq; Obama clearly had no desire to get bogged down in a civil war in Syria. Even in Iraq, though, he would send no combat troops: American planes would pound ISIS targets from the air, while Iraqis—a mix of its national army, various militias, and Kurdish peshmerga—would fight on the ground.

He had two other preconditions for any American role. First, Iraqi Prime Minister Nouri al-Maliki had to go; he’d long been using his political power to exclude and persecute Sunnis. ISIS, a Sunni organization, adroitly exploited public disaffection with Maliki to recruit fighters and supporters. If moderate Sunnis were to be split away from the ISIS jihadists, Iraq needed a more inclusive government. The second precondition was that other countries, especially Sunni Muslim countries, had to join the fight; if this were seen as an American war, it would play into ISIS propaganda.

For a while, the strategy seemed to be … not quite working, but a promising work in progress. Obama and Secretary of State John Kerry used their leverage with other Iraqi leaders and sheiks to pry Maliki from power. A number of Muslim countries—Saudi Arabia, Egypt, Jordan, Qatar, the United Arab Emirates, and, a bit later, Turkey—signed up with the coalition (along with several NATO allies). The first combined-arms campaign—U.S. air strikes supporting a joint assault by the Iraqi army, Shiite militias, and the peshmerga to push ISIS forces away from the Mosul dam—was a marked success.

In a televised address this past Sept. 11, Obama expanded his strategy a little bit—just enough, it seems in retrospect, to trigger his current troubles. He announced that he would now be bombing and strafing ISIS wherever its fighters roamed—in Syria as well as Iraq.

This shift wasn’t as open-ended as it seemed. A senior official stressed, in a preview of the speech, that at least initially U.S. air strikes in Syria would be clustered along the border, to keep ISIS jihadists from moving back and forth between the two countries or from seeking safe haven. At some point, the strikes would expand across Syria, but not yet. Air strikes alone rarely win battles, much less wars. They need to be combined with ground assaults, and the “moderate rebels” of the Free Syrian Army weren’t ready to confront ISIS.

Yet now, just four weeks later, Obama is dropping more bombs on Syria than he ever did, in the course of a day, on Iraq. How did this happen? The way mission creep usually happens: It crept up on him.

In late September, ISIS troops launched an assault on Kurdish territory in northern Syria. For the past few days, they’ve seemed on the verge of conquering the town of Kobani, right on the Turkish border. It’s been fairly predicted that, if the jihadists captured Kobani, they’d murder nearly everyone there. (Shiite Muslims may be infidels in their eyes, but Kurds are beyond the pale.) More than 100,000 residents fled across the border, anticipating the worst. In response, the Turkish army amassed an array of tanks just across from Kobani. American and UAE planes started dropping bombs on ISIS positions just south of the village. The ISIS fighters—who reportedly take cover whenever they so much as hear an airplane overhead—stopped in their tracks.

At this point, in most coalition wars, the allied air commanders would signal ground troops to move in. In this case, that would mean the Turks would roll in the tanks and infantry to envelop and crush the ISIS fighters in their moment of disarray. Something like this happened in the early stages of the 2001–02 Afghanistan war: American planes would drop smart bombs on Taliban positions, then Northern Alliance militiamen would charge in. This sequence of one-two punches led to the Taliban’s defeat much more quickly than anyone had guessed possible.

But an odd thing happened in northern Syria: The Turks didn’t do anything. Their tanks just stood there. Instead, Turkish President Recep Tayyip Erdogan made an announcement. Yes, he acknowledged, Kobani is about to fall. But, he added, he won’t send his own troops into the fray unless the United States does more to help the rebels—the ones many Americans call “moderate rebels”—overthrow Syrian President Bashar al-Assad.

So, let’s review. Here’s Obama, sending American advisers and pilots halfway around the world, spending $10 million a day or more on a fight that has no direct impact on U.S. national security. In the meantime, Erdogan twiddles his thumbs while ISIS fights its way up a hill a mile away from his border. And as the twiddling proceeds, the Syrian Kurds, who have been staving off the jihadi assault, are running out of ammunition.

Kerry is lobbying Erdogan to recalculate his interests. Rifts are reportedly rising between the Turkish president and some of his generals, and Turkey’s Kurdish population has taken to the streets in protest. Meanwhile, to stave off the fall of Kobani (which would probably lead to more incursions across Kurdistan and, by itself, set off a humanitarian disaster of obscene proportions), Obama has dramatically upped the airstrikes. As long as the bombs fall, the ISIS jihadists stay hunkered down; they can’t get any closer to the city.

But, of course, this can go on for only so long. (Even now, ISIS troops are reportedly advancing through the town.) Obama surely knows that he can’t hope to kill all the bad guys through air power alone. Resourceful bad guys—and ISIS seems to be resourceful—adapt to challenges.

Another case in point from the Afghanistan war: Operation Anaconda, which took place in March 2002. Its aim was to clear the al-Qaida holdouts in the Shah-i-kot Valley. Spy satellites and Predator drones took aerial photos of the entire battlefield—a space of 50 square miles—in order to locate every al-Qaida position. Yet when the first U.S. troops dismounted from their assault choppers, they were instantly swarmed by fire from al-Qaida fighters, who had learned the art of camouflage and digging in. A postwar analysis determined that, before the fighting, the satellites and drones had detected fewer than half of al-Qaida’s positions.

Kurdistan offers fewer hiding places than Afghanistan, but the point should be clear: Air power won’t do the trick if the Turks don’t fight on the ground.

So what’s up with Erdogan? He seems to be playing two games. First, he doesn’t want to help the Kurds. He lined up all those tanks along the border for two reasons: to keep ISIS from invading Turkey (an unlikely next-step), but, more, to keep Turkish Kurds from dashing across the line to help their Syrian brethren. Turkish rulers have long been crushing Kurds, whose aspirations for independence, or at least autonomy, have been explicit for even longer. Many of the Kurds eager to go fight in Syria are members of the Kurdish Workers Party, or PKK, which Turkey regards as a terrorist group. To someone like Erdogan, ISIS is a transient nuisance compared with the chronic threat of radical Kurds.

Second, Erdogan is genuinely concerned about the future of Syria—as are the leaders of Egypt, Saudi Arabia, Jordan, Qatar, and the UAE. A case could be made that ISIS won’t be destroyed (it will only be “degraded”) without a political settlement in Syria. ISIS grew amid the political vacuum left by Assad’s demolition war against Sunni rebels, obliterating lively cities and civic institutions. It’s unlikely at this point that the “moderate rebels” could oust Assad or beat ISIS, much less both. A political settlement may require Assad’s arranged departure.

Ousting Assad in Syria is no easy matter—but Erdogan thinks he has leverage to make Obama try. He knows that Turkey is critical to the maintenance of the Muslim coalition against ISIS. He seems to be hoping that Obama is so invested in this fight that he’ll succumb to the pressure—and either let Erdogan continue dominating the Kurds or get more serious about removing Assad.

Erdogan is unlikely to win this game. First, Obama seems genuinely committed to the Kurds. (This tends to happen to any American president who gets to know them.) Second, Obama still seems distinctly uninterested in getting entangled in a Syrian civil war. There’s no percentage in it: Air power alone won’t work; it will take a while—more than a year—to train the “moderate Syrian rebels”; and he is certainly not going to send in American ground troops. The Syrian part of Obama’s anti-ISIS strategy was always a deferral. He seems not to have thought it through, perhaps because he didn’t think he’d have to. It would be hard, and take long, enough to “degrade and destroy” ISIS before he’d have to deal once more with Assad.

He didn’t count on two factors. First, ISIS-in-Iraq and ISIS-in- Syria turn out to be inseparable; it’s hard to fight one without contending with the other. Second, America’s allies in the region—on whom Obama’s strategy depends—have interests that are at times at odds with American interests. This becomes a problem in coalition warfare. ISIS, in fact, gains much of its strength from the fact that the countries arrayed against it—which, together, could win in short order—can’t get their act together; they have too many conflicting interests tearing them apart.

Obama shouldn’t cave in to Erdogan’s blackmail. Even if he werekeen to help overthrow Assad now, there’s no way to do it (unless he can work out some back-room diplomatic deal with Iran, which has the most influence in Damascus—in which case: Yes, do it).

At the same time, though, Obama is locked in to his own commitment. He is wise enough to know that the United States can’t do this sort of thing alone. But he’s also flustered by a central reality of the 21st century: The international system in which we all grew up, the system of the Cold War, has shattered, and nothing has taken its place. There are no real power centers. Nations, even small and medium-sized ones, are freer to pursue their own interests, which often collide with ours. Large nations have less leverage than they once did, and it’s harder to coerce or persuade other nations to put our interests above their own. Obama is in a tight position (and future presidents should take note, because they will be, too): He may have to succumb to mission creep—or slowly, carefully, creep away.