The Islamic State’s so-called “Khorasan province” threatened the Taliban in a video released late last month. The video, which is more than 15 minutes long, features a lengthy speech by an unnamed jihadist (see image above) in front of armed fighters and local villagers. The speaker, citing the Prophet Mohammed, warns that there cannot be two caliphs. If one of the caliphs fulfills the appropriate criteria for being the ummah’s supposed leader — that is, the head of the worldwide community of Muslims — then the other must be vanquished, the speaker says. Although he doesn’t name Afghan Taliban leader...