As the war against the Islamic State expands with airstrikes in Syria, here's what senior national security correspondent Karen DeYoung, national security reporter Greg Miller, White House correspondent Katie Zezima and foreign affairs writer Ishaan Tharoor have to say about what it all means. (Jason Aldag/The Washington Post)

After spending nearly six years of his presidency installing a series of constraints on U.S. counterterrorism operations, President Obama has launched a broad military offensive against Islamist groups in Syria that stretches the limits of those legal and policy enclosures.

The barrage of airstrikes was aimed mainly at a militant group, the Islamic State, that is no longer among the al-Qaeda “associates” envisioned by the military authorization passed after the Sept. 11, 2001, attacks. The group is not even suspected of planning attacks against the United States.

The unfolding U.S. air campaign has employed weapons — including dozens of 3,000-pound Tomahawk missiles launched from U.S. warships — that have flattened targets in ways destined to test Obama’s doctrine requiring “near certainty” that no civilians be killed.

Obama said the assault was a message “to anyone who would plot against America and try to do Americans harm that we will not tolerate safe havens for terrorists.” Pentagon officials described the strikes as a successful opening to a long-term campaign.

The opening salvo included a smaller and separate mission more in line with Obama’s counterterrorism playbook: a flurry of strikes against an al-Qaeda cell said to be “nearing the execution stage” of attacks against America or Europe.

But overall, the initial dimensions of the assault put the United States on a significantly different counterterrorism course than Obama envisioned last year, when he delivered a speech describing the nation’s security landscape as returning to pre-Sept. 11 normalcy.

“We must define the nature and scope of this struggle, or else it will define us,” Obama said in a May 2013 speech at the National Defense University, outlining an array of new limits, including “respect for state sovereignty.”

“There are a lot of lines that he’s drawn in the sand. Just about every one of which he seems to have crossed now,” said Jack Goldsmith, a Harvard University law professor and senior Justice Department official in the Bush administration, who attributed the outcome in part to the nature of Obama’s job.

“The reality is that security threats are his first responsibility,” Goldsmith said. “Between past statements and pretty-sounding principles on the one hand, and the reality of security threats on the other, every president will always address the security threats and discard the principles.”

Obama administration officials disputed that characterization and said the unfolding offensive in Syria is in line with the president’s broader objectives to keep U.S. forces out of ground wars, place more pressure on foreign allies to confront overseas threats, and rely on lethal U.S. capabilities only as a last resort.

In remarks Tuesday, Obama stressed the involvement of other countries, including Saudi Arabia and Jordan, and described the danger posed by the Islamic State — which has seized territory in Iraq and Syria and beheaded two American journalists — as one he could not ignore.

But eight of the 22 airstrikes that began Monday night Washington time were aimed at targets associated with an alleged al-Qaeda cell in Aleppo called the Khorasan group, an organization so obscure that U.S. officials had never even mentioned it by name until a week ago.

Before getting on a flight Tuesday to New York City, where he was expected to speak about climate change, President Obama addressed the airstrikes that the U.S. launched Monday night against the Islamic State in Syria. (AP)

By Tuesday, officials across the Obama administration and U.S. military were describing the group as an “imminent threat” to this country.

U.S. intelligence officials described the group as a unit of Jabhat al-Nusra, al-Qaeda’s affiliate in Syria, that was formed specifically to plot attacks against Western targets. “These are operatives who are quite seasoned,” with experience in Pakistan, Yemen and other conflict zones, a senior administration official said.

U.S. officials provided varying descriptions of the alleged Khorasan plot, with some saying that heightened airport security reflected broad concern about the group’s expertise with explosives and efforts to recruit Westerners, rather than a plan in motion.

Sen. Angus King (I-Maine), a member of the Senate Intelligence Committee, declined to discuss specifics of the threat tied to Khorasan, but he described strikes against the group as warranted. “Their business is planning strikes against the West,” he said.

Obama’s decision to approve a Syria campaign required White House lawyers to scour international and domestic law and the administration’s own counterterrorism guidelines for ways to justify operations that might exceed the narrow limits Obama had set on lethal action outside U.S. war zones.

In particular, the administration had called for repeal of the two congressional Authorizations for Use of Military Force (AUMF) passed since al-Qaeda’s Sept. 2001 attacks. A 2001 AUMF against al-Qaeda and its associates was seen as overly broad and obsolete in the face of al-Qaeda’s near-demise in Pakistan and Afghanistan and the development of new terrorism threats with no relation to the World Trade Center attacks. A 2002 AUMF on Iraq had been directed at Saddam Hussein, who no longer existed.

Yet those two authorizations have formed the basis of the administration’s justification for its current actions in both Iraq and Syria, along with an assertion of the president’s constitutional power to protect American citizens and national security.

In an NBC interview Tuesday, Antony Blinken, deputy national security adviser, said the Syria strikes were justified under “a doctrine of collective self-defense,” because Iraq had asked “the United States and other countries to act against ISIL because ISIL in Syria threatens them.” ISIL is another acronym for the Islamic State. Samantha Power, the U.S. ambassador to the United Nations, offered the same justification in a letter to U.N. Secretary General Ban Ki-moon.

Earlier in the day, as it strived to cover all international and domestic justification bases, the White House sent Congress notification of its actions under the 1973 War Powers Resolution.

A senior administration official, briefing reporters Tuesday under White House-imposed conditions of anonymity, said that “the president had the authority under the 2001 AUMF” to strike in Syria because the Islamic State has its roots in an al-Qaeda affiliated group founded in Iraq in 2003, even though it split from al-Qaeda earlier this year.

Congress did not intend “to remove the president’s authority to use force against this group simply because it had a disagreement with the al-Qaeda leadership,” the official said, a position that is at odds with earlier interpretations of the 2001 law.

Obama is unlikely to be challenged on the issue at a moment when there is broad support in Congress for strikes against the Islamic State and an expanded effort to arm and equip moderate rebels in Syria.

An increasing number of lawmakers have raised concern about the administration’s elastic interpretations of the AUMF and have introduced bills designed to address that measure’s perceived flaws.

A measure sponsored by Sen. Timothy M. Kaine (D-Va.) would repeal the 2002 Iraq authorization and approve action against the Islamic State for one year only, with limitations on ground troops and on applicability to any other so-called “associated forces.”

“Ultimately, this is about a precedent for the future,” Kaine said in a speech Tuesday at the Center for American Progress. “If Congress allows this president to begin this campaign against ISIL . . . we will have created a horrible precedent that future presidents will no doubt use.”

In the House, a bill introduced by Rep. Adam B. Schiff (D-Calif.) would repeal the Iraq AUMF. It would pass a new authorization narrowly allowing action against the Islamic State in Iraq and Syria and sunset that measure, along with the 2001 authorization, after 18 months.

By relying on the 2001 al-Qaeda authorization, Schiff said in an interview, the administration is “putting the best legal arguments on a very weak case.” They “would probably like a new authorization that repeals the old ones and sets out a new authority,” he said, “but I’m not sure they’re confident it can be done in such a dysfunctional Congress.”

Adam Goldman and Julie Tate contributed to this story.