U.S. President Barack Obama delivers a live televised address to the nation on his plans for military action against the Islamic State. (Pool/Reuters)

PRESIDENT OBAMA promised Wednesday night to meet the terrorist threat in Iraq and Syria “with strength and resolve.” His commitment to “ultimately destroy” the Islamic State was bold and necessary. But it was also incomplete.

A strategy built exclusively on killing terrorists will have no end. The United States must also help Iraqis and Syrians build institutions — police, courts, schools, businesses that offer jobs — so that terrorist organizations do not emerge again as soon as Americans look away. The term “nation-building” understandably is out of favor: It has a hubristic quality that rings especially false to Americans who have watched their countrymen die in Afghanistan and Iraq for so many years. America cannot “build” Iraq; Iraqis must do that. But Americans, if they want to beat back the Islamic State, must also be committed to helping Iraqis build Iraq.

On one level, Mr. Obama’s speech indicated that he understands this. He said that Iraq needed to form “an inclusive government” and that he would pursue “the political solution necessary to solve Syria’s crisis once and for all.” He said the United States is helping Muslim communities around the world “fight for opportunity, tolerance, and a more hopeful future. . . . We stand for freedom, for justice, for dignity.”

But the tactics he prescribed and the models he cited — of U.S. policy in Somalia and Yemen — do not match either his lofty rhetoric or, more pragmatically, his political goals. In those two nations, the United States kills terrorists from the air while training local forces to hunt them on the ground. But Somalia remains, despite the valiant effort of many Somalis, a failed state, and Yemen is hardly a picture of progress. At best, unless conditions improve for most people, the United States will be there forever, killing terrorists as they appear. It’s far from clear such a policy will be sustainable. “[T]he ‘Yemen model’ has not been as successful as the White House claims; indeed, it is in danger of collapse,” al-Qaeda expert Katherine Zimmerman wrote on the opposite page a couple of months ago. “Attempting to replicate it in much more challenging conditions in Iraq and Syria will almost certainly fail.”

“We cannot do for Iraqis what they must do for themselves,” Mr. Obama said, and that is as true of institution-building as it is of killing terrorists. But U.S. assistance is necessary for both endeavors, for two reasons. If Mr. Obama tells the locals that he is dispatching U.S. troops only to kill those who might someday threaten the United States, those troops are unlikely to win anything but grudging or mercenary cooperation from their local partners.

Moreover, unless those nations succeed, this cycle will never end. President George W. Bush’s surge, accompanied by intensive institution-building efforts, helped beat al-Qaeda in Iraq back to near-insignificance. But when the United States withdrew from Iraq, and stayed aloof from the metastasizing civil war in Syria, the terrorists came back.

A full-fledged strategy would not require U.S. combat troops, but it would take more than the 475 additional advisers Mr. Obama committed Wednesday. And it would take an understanding that a sustained, long-term commitment would be more fruitful over time than cycles of intervention and withdrawal.