WASHINGTON — Nearly six years into a presidency devoted to ending U.S. wars in the Muslim world, President Barack Obama faced the nation Wednesday night to explain why he has decided to engage in a new one.

Obama did not describe his authorization of direct military action to defeat the Islamic State terror group as a conventional war. To the contrary, in a prime-time address from the White House, he sharply contrasted his use of targeted but limited American force with the large scale air-and-ground invasions launched by his predecessor, George W. Bush.

“This effort will be different from the wars in Iraq and Afghanistan,” he said in prepared remarks.

But make no mistake: Obama’s escalation of airstrikes and the use of U.S. personnel to help “degrade and destroy” the extremist Sunni group represents a major setback for a commander in chief whose early international appeal was built on a pledge to remove the United States from “permanent war footing.”

“How did this group that came in determined to remedy the Bush administration’s overreach . . . end up embroiled in a far more open-ended conflict that has just as far-reaching consequences?” said Rosa Brooks, a former Obama administration official who served at the Pentagon from 2009 to 2011. “This is a legacy issue for him.”

Senior advisers have repeatedly said that the unexpected course of the Arab Spring greatly limited their ability to shape events in countries such as Syria. But whatever the source of unrest, it is clear that Obama was either naive to promise a new chapter in post-9/11 foreign policy or simply failed to deliver on that vision.

The night before Obama planned to visit the Pentagon on the 13th anniversary of the 9/11 terrorist attacks, his task was to reconcile the concerns of a public weary of war but, polls show, increasingly supportive of military action to stem the threat of the Islamic State.

Already, the Pentagon has carried out 154 airstrikes on Islamic State forces in Iraq over the past month. The president has pledged not to send combat troops to the fight, but 1,043 U.S. service personnel are supporting the effort in Iraq – three years after Obama withdrew the final U.S. troops and declared an end to the war there.

“You are the first class to graduate since 9/11 who may not be sent into combat in Iraq or Afghanistan,” Obama told graduates of the U.S. Military Academy in West Point, New York during a commencement speech in May that aimed to define his foreign policy as one that employs lethal force in strategic ways but avoids drawn-out campaigns on foreign soil. Obama has announced that all but a residual force of 9,800 U.S. troops will leave Afghanistan after this year.

Obama’s challenge in persuading the American public to support his strategy against the Islamic State has less to do with how many troops are in the fight than with “what are the strategic objectives of the United States?” said Tamara Cofman Wittes, a Middle East expert at the Brookings Institution who worked at the State Department from 2009 to 2012.

“You can say ‘no boots on the ground,’ but that does not really solve their anxiety,” she said. “They want to know what defines the end of our engagement.”

The president and his advisers have acknowledged in meetings with congressional leaders and foreign policy experts that the campaign to defeat the Islamic State will take years and will probably extend, in one form or another, beyond the day Obama leaves office in January 2017.

It is not a legacy the president expected to leave. Less than a year after taking office, Obama delivered an address in Oslo as he accepted a Nobel Peace Prize, awarded for what the prize committee said was “his extraordinary efforts to strengthen international diplomacy and cooperation between people.”

Since then, Obama – to the deep dismay of civil liberties advocates – has attempted to define a new notion of how to use U.S. military might to take the fight to the enemy. His counterterrorism strategy, with its reliance on the lethal force of unmanned Predator drones and secret terrorist kill lists, has limited casualties to but outraged those who expected him to depart more fully from his predecessor.