The most effective counterterrorism force in the world is Israel’s Sayeret Matkal—“the Unit.” A highly secretive special-operations brigade, it has taken on mythic status with swift, surgical victories in Egypt, Lebanon, Jordan, and, perhaps most famously, Entebbe, Uganda, where its commandos freed 103 Jewish hostages after a 1976 hijacking. Probing the Unit’s shadowy history, Rich…