The downing of a Russian passenger plane over Egypt’s Sinai Peninsula last October, for which the Islamic State (also known as ISIS) claimed responsibility, may ultimately prove more consequential than the horrific attacks in Paris and San Bernardino, California, that followed. Western security officials had long worried that their countries’ own citizens would conduct attacks after returning home from Iraq or Syria or strike out as “lone wolf” terrorists. But the Russian plane crash, which killed 224 people, was caused by a different beast: neither lone wolves nor ISIS itself but an ISIS affiliate that had pledged its loyalty to