President Obama outlined a four-step plan on combating the threat of the Islamic State. Here are the highlights from his speech. (Nicki DeMarco and Ashleigh Joplin/The Washington Post)

Nearly six years into a presidency devoted to ending U.S. wars in the Muslim world, President 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, casting the strategy as a “counterterrorism campaign” similar to U.S. intervention in Somalia and Yemen. “It is consistent with the approach I outlined earlier this year: to use force against anyone who threatens America’s core interests, but to mobilize partners wherever possible.”

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 and 475 more are on the way — 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 West Point graduates 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 group has less to do with how many troops are in the fight than answering the question, “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.

President Obama said the U.S. will work with a "broad coalition" of foreign partners to combat the Islamic State in his public address on Wednesday, Sept. 10, 2014. (The Associated Press)

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 called “his extraordinary efforts to strengthen international diplomacy and cooperation between people.”

The president spoke about the search, among philosophers and statesmen, for the terms that would define a “just war” — but concluded that such a conflict was “rarely observed.”

“I do not bring with me today a definitive solution to the problems of war,” Obama said. “What I do know is that . . . it will require us to think in new ways about the notions of just war and the imperatives of a just peace.”

Since then, Obama — to the deep dismay of civil liberties advocates — has attempted to redefine 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 administration terrorist kill lists, has limited casualties to U.S. troops while outraging those who expected him to depart more fully from the tactics of his predecessor.

White House aides point to successes — most famously, the Navy SEALs raid that killed Osama bin Laden in Pakistan in 2011 and, this month, a Pentagon airstrike that killed a high-level leader of an al-Qaeda-linked group in Somalia.

“The thinking about how to deal effectively with [the Islamic State] is informed by our experience with al-Qaeda in the ­[Afghanistan-Pakistan] region,” said Michèle Flournoy, who was undersecretary of defense for policy from 2009 to 2012. “There were a lot of lessons learned and best practices of how to prosecute a long-term counterterrorism campaign to degrade and defeat senior leadership.”

In cases where the president has chosen to employ more sustained force, such as his intervention in Libya in 2011 with U.S. airstrikes that helped oust dictator Moammar Gaddafi, he has attempted to abide by a set of principles, aides say. They include building an international coalition and training and supporting local troops as the primary fighting forces on the ground.

Obama has been criticized by Republican hawks for “leading from behind.” The descent into chaos in Libya since Gaddafi was killed in 2011 and the broader failure of the Arab Spring to advance democracy in the Middle East and North Africa have also dampened White House claims of progress.

For the president, the desire not to rush headlong into sustained conflict has been captured in a West Wing mantra used by Obama and his aides: “Don’t do stupid stuff.” That mind-set can be seen in Obama’s deliberate response to the Russia-backed incursion of rebel forces in eastern Ukraine, in which the administration has sought to lead a coalition that has applied economic sanctions against Russia without supplying lethal military weapons to Ukraine’s ruling government.

“This is American leadership. This is American strength,” Obama said at West Point in May, referring to international sanctions that opened channels for negotiations with Iran over that country’s nuclear program.

Still, Obama’s ambivalence over how to deal with the threats emanating from Syria has been on full display for months.

A year ago to the day, Obama addressed the nation in prime time to make the case for targeted airstrikes against Bashar al-Assad’s regime after evidence emerged that the Syrian president had crossed Obama’s “red line” and used chemical weapons against civilians in his country’s civil war.

Even though Obama had concluded that strikes were necessary, at the last moment he changed his mind about ordering the military to act. Instead, he used his speech to request formal congressional support, which never came.

On Wednesday night, he returned to TV, this time to tell the American public that the time has come — whether Congress likes it or not — to send U.S. warplanes into battle once again.