On CBS’s “Face the Nation” Sunday, President Barack Obama defended his decision to send another 1,500 American trainers and advisers to Iraq, arguing that it represented the next “phase” of the campaign against the Islamic State. “What we knew was that phase one was getting an Iraqi government,” he said. “And so now what we've done is rather than just try to halt ISIL's momentum. We're now in a position to start going on some offense.” That offense will rely on Iraqi ground troops; American troops will not be in combat, Obama said. It’s his latest promise about the U.S. military’s involvement in Iraq and Syria that he may not be able to keep.

When Obama announced the policy in September, he guaranteed the country that American soldiers would not be fighting in Iraq or Syria. “I want the American people to understand how this effort will be different from the wars in Iraq and Afghanistan,” he said last week. “It will not involve American combat troops fighting on foreign soil.” The key word here is “combat.” Obama never promised to keep boots off the ground, so the 1,500 additional troops—bring the total number in Iraq to 3,000—is keeping with that original promise.

These troops will train Iraqi ground troops and work with Iraqi military commanders to push back the Islamic State, with the hopes of retaking land currently controlled by the terrorist organization including the city of Mosul. Is the Iraqi military capable of that, even with support from U.S. airstrikes? That’s unknown but many U.S. military commanders are skeptical of the strategy. What happens if the Iraqi military is unable to push back the Islamic State? Will Obama give up on the second phase of his strategy and allow the Islamic State to retain control over broad swaths of Iraqi territory? This would force Obama to break one of two promises: Either he will not “degrade and destroy” the Islamic State or he will be forced to use U.S. ground troops in combat situations against them. (Obama reiterated his commitment to destroying the terrorist group on Sunday, as well.)

U.S. military commanders have been clear that they will advise using U.S. troops in combat operations if needed. In the New York Times Monday, for instance, General Ray Odierno, the Army chief of staff, said, “Over time, if [Iraqi ground troops are] not working, then we’re going to have to reassess, and we’ll have to decide whether we think it’s worth putting other forces in there, to include U.S. forces.” Would Obama side with his generals in that situation? We have no idea. All we now know is what the president said Sunday: “We will provide [the Iraqis] close air support once they are prepared to start going on the offense against ISIL, but what we will not be doing is having our troops do the fighting.”

This hole in Obama’s strategy should worry Americans. Since the administration has not explained its plan if Iraqi ground troops fail to drive back the Islamic State, the public—and Congress, if it ever decides to involve itself in the strategy—cannot understand the risks involved with the current military engagement. The U.S. could end up with thousands of troops fighting in Iraq.

If we’ve learned anything over the past decade, it should be that wars in the Middle East are unpredictable. It’s impossible for the president to make sweeping promises about the U.S.’s future foreign policy. We simply don’t know how the current military engagement will evolve. Obama doesn’t seem to have forgotten this lesson. In an answering a question Sunday about whether more ground troops will be need in Iraq, he said, “You know, as commander-in-chief, I'm never going to say never.” If only he would say the same about all parts of his strategy.