Muhammad bin Salman Al-Saud and Muhammad bin Zayed Al-Nahyan, the respective princely strongmen of Saudi Arabia and the UAE, have torn up the old rules. They have spurred game-changing economic master plans, presided over vast anti-corruption crackdowns, tackled entrenched religious forces, and overseen the mass arrest of critics. In parallel, they also appear to have replaced the old 'sheikhly' consensus systems of their predecessors with something more autocratic, more personalistic, and perhaps even analytically distinct. These are the two wealthiest and most populous Gulf monarchies, and...