In this June 16, 2014, file photo, supporters of the Islamic State of Iraq and Greater Syria chant slogans as they carry the group's flags in Mosul, IraqSTR—AP What he found in Mosul doesn't bode well Jürgen Todenhöfer, the first Western journalist to be granted access to territories controlled by the Islamic State of Iraq and Greater Syria (ISIS), has returned with a warning: the terrorist group is “much stronger and much more dangerous” than its adversaries understand. The veteran German writer and journalist is back in his home state after traveling through ISIS territories, just months after the extremist group began killing captive foreign workers and journalists. In Todenhöfer’s first interviews about the trip with German-language media, translated by the U.K. Independent, he presents ISIS as having achieved its namesake goal: an Islamic State — or rather, a collection of claimed lands hewed together by an audacious, baffling zealotry that will challenge efforts to beat...