The American government is also citing a Sept. 20 letter from Iraq’s minister of foreign affairs, Ibrahim al-Jafari, to the United Nations complaining that the Islamic State was attacking Iraq from its havens and saying that it had requested the United States’ assistance in defending itself.  Iraq has “requested the United States of America to lead international efforts to strike ISIL sites and military strongholds, with our express consent,” the Iraqi letter said. “The aim of such strikes is to end the constant threat to Iraq, protect Iraq’s citizens and, ultimately, arm Iraqi forces and enable them to regain control of Iraq’s borders.”  Two legal scholars, Jack Goldsmith of Harvard Law School and Ryan Goodman of the New York University School of Law, said the United States appeared to be on solid ground by invoking the argument of collective self-defense of Iraq, but that the notion that Syria’s sovereignty could legally be violated because it was “unable or unwilling” to suppress the threat would be more controversial. While the United States has long invoked that argument in various contexts, many international law scholars disagree with it, they said.  The United States is also asserting a right to defend its own personnel in Iraq from the Islamic State. American officials said this right, which was not asserted in the letter to Mr. Ban, should be understood as supplementary authority to helping Iraq defend itself directly.  Administration officials have said that as a matter of domestic law, they believe that the United States has statutory authority to attack the Islamic State under Congress’s 2001 authorization to fight Al Qaeda. They also believe that Congress’s 2002 authorization of the Iraq war could provide an alternate source of such authority. The United States has been bombing Islamic State forces in Iraq since August.  Both congressional authorizations provide legal authority for the strikes in Syria, too, the officials contended, because of the Islamic State’s history of ties to Al Qaeda — notwithstanding the fact that the two groups recently split. And, they said, the 2002 Iraq war authorization can be read in part as promising to help foster a stable, democratic government in Iraq, which would include defending it from terrorist attacks.  In May 2013, President Obama announced a new policy for targeted killings under which the United States would generally strike only at specific individuals who were deemed to pose a “continuing and imminent threat” of attacks on Americans. The policy appeared designed to foreclose the possibility of so-called signature strikes, which target groups of people whose identities are unknown but whose patterns of life suggest that they are members of militant groups.  Neither the strikes targeting the Islamic State nor those targeting Khorasan were based on any individualized, case-by-case analysis that a specific person in the strike zone posed a continuing and imminent threat to the United States, the officials said. Rather, the United States was hitting members of the groups based on their status as part of an enemy force.  The officials said the May 2013 policy for targeted killings did not apply to the broader armed conflict now underway in Iraq and Syria.  Somini Sengupta reported from the United Nations, and Charlie Savage from Washington.  A version of this article appears in print on September 24, 2014, on Page A19 of the New York edition with the headline: U.S. Invokes Defense of Iraq in Legal Justification of Syria Strikes. Order Reprints| Today's Paper|Subscribe
The airstrikes were carried out without the permission of the Syrian government or the United Nations Security Council.