Why can’t these jihadis just get along and turn their fire jointly against the United States? The deep reason is ideological. The Taliban’s founder, Mullah Omar, styled himself “Emir of the believers,” a term equivalent in Islam’s classical period to caliph. Certain rules apply to caliphs, and foremost among them is that there can be only one at a time. Others include the requirement that the caliph be physically whole and descended from the tribe of the Prophet Muhammad. Like most of the original generation of the Taliban, Omar was partially dismembered (missing an eye, in his case), and descended not from an Arab tribe but from a Pashtun clan. Oh, and another requirement of the caliph is that he must be alive. Omar died in 2013, but the Taliban pulled a Weekend at Bernie’s and lied about it for two years. The Islamic State, noting these deficiencies, named its own caliph in 2014 when it took control of parts of Syria and Iraq, and relentlessly mocked the Taliban for being apostate hillbillies out of compliance with even the most basic elements of Islam.

One cannot easily forgive or retract insults such as these. Nor can one take back what followed: an all-out military challenge by the Islamic State to the Taliban’s supremacy as Afghanistan’s Islamist guerrilla force. These two factions fought a war—and even though the Taliban had a decades-long head start, the competition was a close one in certain areas, particularly eastern Afghanistan. The U.S. intervened to help the Taliban, under the theory that at least the hillbillies were interested only in Afghanistan, whereas the Islamic State would blow up every statehouse and disco on Earth if it had the chance.