In a statement Thursday night, President Obama said he has authorized military strikes on Islamic State militants if they continue their advancement on the city of Irbil, where military personnel and the U.S. consulate are located. (AP)

President Obama’s decision Friday to launch airstrikes in Iraq reflected an important shift for a president who had spent months making the case for how the United States could achieve its foreign policy objectives without the use of force. His conclusion: Sometimes there is no substitute for military might.

Both Obama and his top aides have emphasized that any bombing in Iraq where tens of thousands of Yazidis are trapped and surrounded by Islamist militants, who also threaten American diplomats in the Kurdish regional capital of Irbil, is limited in scope. But it reflects a return to his response to the crisis in Libya three years ago, marking a departure from the foreign policy approach the administration has espoused for much of its second term.

“I see this as a recognition of the costs of constraint,” said Jon B. Alterman, who directs the Middle East program at the Center for Strategic and International Studies, adding that when it comes to the question of U.S. military intervention abroad, “the determination to not intervene has turned into a reluctant, limited involvement.”

While Obama has sought in the past to turn to Congress and America’s allies for support when considering military action in the Mideast — most notably, in Syria last year — this time it was clear the president was on his own.

“This is us and nobody, which is often the case,” said James F. Jeffrey, who served as U.S. ambassador to Iraq between 2010 and 2012 and is now a distinguished visiting fellow at the Washington Institute for Near East Policy.

A handful of lawmakers from both parties issued largely supportive statements about Obama’s decision, saying the United States needed to act not only to prevent the extermination of the Yazidi population but to halt the advance of the Islamic State, an al-Qaeda offshoot that has taken over much of northern and western Iraq. But some Republicans still criticized the president for responding too slowly to the Islamist militants’ advances.

House Speaker John A. Boehner (R-Ohio) issued a statement Thursday night saying he was “dismayed by the ongoing absence of a strategy” in Iraq.

“Vital national interests are at stake, yet the White House has remained disengaged despite warnings from Iraqi leaders, Congress, and even members of its own administration,” Boehner said. “Such parochial thinking only emboldens the enemy and squanders the sacrifices Americans have made.”

Congress has shown little willingness to exercise its authority under the War Powers Act when it comes to conflicts across the globe, whether they are in Syria, Iraq or Ukraine, leaving the White House in charge. Duke University political science professor Peter Feaver, who served as a National Security Council special adviser under George W. Bush, said there is a “structural asymmetry” between lawmakers’ and the president’s role when it comes to these questions.

“Congress has a vital and constructive role to play in foreign policy, but at the end of the day there is only one President,” Feaver said. “And that means that if they want to, 535 members of Congress get to be pigeons, but the president has to be the sculptor. He has to make the statue. They can dump on the statue.”

The president’s discomfort with the idea of authorizing force in Iraq — the war he worked to end during his first term — is clearly visible. Jeffrey called the situation Obama’s “worst nightmare,” adding the president’s words and body language reflected his “genuine, extreme reluctance” to get entangled in another military conflict overseas.

“The problem is his very strong belief is clashing with the reality on the ground,” Jeffrey added. “This al-Qaeda offshoot will not be stopped without the use of American military power.”

A senior administration official said in a late-night phone call with reporters Thursday that the Islamic State’s multipronged attack over the weekend “was swift; it was effective.” The gains the group has made in Syria and Iraq, the aide added, require “a level of sophistication in terms of a military response.”

And the fact that the threat had reached the Kurds — longtime American allies, whose government has not been plagued by the same problems that have dogged Iraqi President Nouri al-Maliki — made U.S. intervention almost inevitable.

On one level, the administration’s use of airstrikes in the face of a humanitarian crisis is nothing new. Not only did the United States join Britain and France in launching strikes against Libya in 2011, President Clinton exercised that option twice in the 1990s, in both Bosnia and Kosovo.

But Obama’s intervention, coming after major gains by the Islamic State, raises questions about how much more deeply the administration will have to intervene in the coming weeks. Alterman noted that in his past discussions with senior administration officials about the possibility of airstrikes in Syria, they repeatedly raised the concern that “it would ‘open the door’ to further involvement as if they had limits that were both natural and self-reinforcing. I’m not sure that in practice those limits exists.”

Dennis Ross, a counselor at the Washington Institute who served as special assistant to Obama from 2009 to 2011, said one “bounded” objective the administration could pursue is shoring up Kurdistan because it has a well-functioning government and a populace willing to fight for itself. But he cautioned that the United States must not only contain but roll back the militants, because they are projecting an image of success “that in of itself creates an attraction” to possible future recruits.

When faced with the prospect of launching airstrikes in Syria, Obama has publicly and privately posed the question, “And then what?” to convey the risks that arise in the aftermath of exercising armed force. Both Ross and Feaver noted that while the president had proved adept at identifying the risks of action, he was less skilled when it came to appreciating the dangers of inaction.

“You do have to be able to answer that question, ‘And then what?’ ” Ross said. “But you also have to answer the question, ‘What if we don’t do it?’ And we are seeing what happens when we don’t do it.”