An image grab taken from a video uploaded on May 18 by Aamaq News Agency, a Youtube channel, reportedly shows Islamic State fighters in a street in Ramadi, Iraq, a day after it was captured. (Agence France-Presse via Getty Images)

Kimberly Kagan is president of the Institute for the Study of War. Frederick W. Kagan is director of the Critical Threats Project at the American Enterprise Institute.

The seizure of Ramadi on Sunday leaves President Obama’s strategy against the Islamic State in ruins not only in Iraq but also throughout the Muslim world. It means that the Iraqi security forces will almost certainly not be able to recapture Mosul this year and, therefore, that the Islamic State will retain its largest city in Iraq. Worse, it gives the group momentum again in Iraq even as it gains ground in Syria and expands in the Sinai, Yemen, Afghanistan and elsewhere. This defeat was avoidable. Neither the Islamic State nor any other al-Qaeda offshoot has ever taken a major urban area actively defended by the United States in partnership with local forces. This is what happens when a policy of half-measures, restrictions and posturing meets a skillful and determined enemy on the battlefield. If the president does not change course soon, he will find that his legacy is not peace with Iran and ending wars, but rather the establishment of a terrorist state with the resources to conduct devastating attacks against the United States and a region-engulfing sectarian war.

Obama reacted slowly and reluctantly to the initial Islamic State surge last June from Syria into Mosul and then down the Tigris toward Baghdad. He authorized U.S. air support to assist the defense of the Kurdish capital of Irbil in August and eventually deployed first a few hundred and then a few thousand U.S. advisers. He did not allow those advisers to fight alongside the Iraqi units they were assisting. U.S. airstrikes have destroyed many fixed Islamic State targets and killed its fighters by the thousands since then, mainly in Iraq, but have allowed the group to retain a haven in Syria and even to maneuver freely within Iraq.

The Islamic State maneuver that led up to the fall of Ramadi was sophisticated and many weeks in the making, as a recent publication from the Institute for the Study of War shows. It entailed diversionary attacks in Baiji and Garma, a prison break in Diyala, attacks against pilgrims in Baghdad and raids near Ayn al-Asad air base west of Ramadi, a major hub of U.S. forces and Iraqi training. It was accompanied by a coordinated offensive around Deir ez-Zor, in Syria, that could give the group the ability to operate all along the Euphrates and toward Damascus as well. Numerous Islamic State fighters moved across Iraq and Syria. Although they leveraged poor weather that impedes U.S. reconnaissance, such activity must have created a signature that a properly resourced U.S. force in the region would have detected, and it certainly created a proliferation of targets on the ground for combinations of attack aviation and ground maneuvers — had those resources been available and allowed to operate freely. U.S. military power, properly employed and resourced, can thwart these kinds of maneuvers. The fall of Ramadi was unnecessary and avoidable.

It is also a major strategic setback. The president’s strategy has been to support the Iraqi security forces in retaking territory lost to the Islamic State last summer and then, in some unspecified manner, turn to confronting it in Syria (probably in the next president’s term). Statements by U.S. and Iraqi leaders this spring made it clear that their plans involved holding in Anbar while focusing on a major operation to retake Mosul sometime this year. That operation would have been an essential precondition for the liberation of the rest of Anbar and for any subsequent operations against the Islamic State in Syria. But discussions about whether the retaking of Mosul would lead to the immediate collapse of the group now appear to have been premature. The disaster in Anbar, along with the fight for Tikrit precipitated by Iranian-backed Shiite militias that ultimately required the diversion of U.S. and Iraqi assets, has certainly derailed any campaign aiming at an early reconquest of Mosul.

Setbacks against the Islamic State in Iraq might not be so devastating if the United States and its allies were on the offensive against that group elsewhere. The president’s plan, unfortunately, confined our efforts almost exclusively to Iraq. In the meantime, the group has managed to gain adherents in the Sinai, Yemen, Libya, Afghanistan, Pakistan and even further abroad, as part of its strategy to remain in Iraq and Syria and expand the caliphate. Had the Islamic State been dealt a rapid and crushing blow in Iraq, one might have hoped for a collapse in support for the organization and the dwindling of these various movements, all of which were preexisting organizations that swore allegiance to the Islamic State opportunistically in the hope that they would prove to be early backers of what Osama bin Laden liked to call “the strong horse.” The Islamic State’s success against the United States in Iraq makes the group look, indeed, like a strong horse and is likely to strengthen its efforts to recruit individuals and groups to its ranks. The fall of Ramadi is a major strategic defeat for the United States and an important victory for the Islamic State, even if it proves ephemeral.

The White House is no doubt abuzz with recommendations, many probably counseling avoiding being sucked further into Iraq. Such recommendations would be completely wrongheaded. We are already sucked into Iraq for the simple reason that an enemy that has claimed credit for lone-wolf attacks in the United States and Australia (which it quite probably inspired, although did not direct) is entrenched there, defeating our local partners, and threatens to establish a quasi-state that controls several large cities. Even at this stage, however, the Islamic State remains unable to stand against even a limited deployment of U.S. military forces if those forces are properly resourced and allowed to operate against the enemy. A few thousand additional combat troops, backed by helicopters, armored vehicles and forward air controllers able to embed with Iraqi units at the battalion level, as well as additional Special Forces troops able to move about the countryside, would certainly prevent further gains. They could almost certainly regain Ramadi and other recently lost areas of Anbar, in cooperation with local tribes. They might be able to do more.

The choice facing Obama is not between a massive deployment of hundreds of thousands of troops and a tightly constrained mission of under-resourced forces. It is, rather, between the serious application of a limited amount of U.S. military power and the establishment of a terrorist state. We submit that it is hard to imagine a serious policy discussion that concludes by favoring the latter outcome.