ISIS’s slick propaganda videos show beheadings and immolations, borrowing motifs from horror movies to inspire revulsion and fear. Its claim of credit for last week’s coordinated terror strike in Paris, which involved seizing a concert hall and gunning down civilians as retribution for French airstrikes in Syria, and for the downing of a passenger plane filled with Russian civilians, are an extension of the square of punishment beyond the group’s original borders, as well as an expression of its grand ambitions, say terrorism experts and those who have fought ISIS on the ground since it emerged from the wreckage of Syria and Iraq.