Obama is reportedly claiming that he has the legal authority to make war on the Islamic State under the authorization of force passed by Congress after the September 11 attacks. But the key language of that authorization reads: “That the President is authorized to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons.”
The Islamic State didn’t plan, authorize, commit, or aid the attacks, and didn’t harbor those who did. Unless there’s some better legal argument lying around, Obama ought to go to Congress for a new authorization.