Nature might abhor a vacuum, but empires love them. The busy empire-builders in Tehran have spent the last two decades extending their reach into power vacuums in Iraq and Syria as part of their attempt to recreate at least part of the old Persian Empire that once covered almost the entire Middle East. Funding the Houthi rebels in Yemen keeps the Saudis conveniently distracted, too, as well as giving Tehran a way to attack the shipping that Western economies rely upon. Hamas and Hezbollah in Gaza and Lebanon respectively are Iran's imperial outposts even further west — and the Arab...