The Obama administration has offered a wide range of assessments of the threat to U.S. national security posed by the extremists who say they've established a caliphate, or Islamic state, in an area straddling eastern Syrian and northern and western Iraq, and whose actions include last week's beheading of American journalist James Foley. [...] there is broad agreement across intelligence and law enforcement agencies of the immediate threat from radicalized Europeans and Americans who could come home to conduct lone-wolf operations. On Friday, Britain raised its terror threat from "substantial" to "severe," its second highest level, citing a foreign fighter danger that made a terrorist attack "highly likely." [...] nearly every senior national security official in the U.S.