Following Israel’s assassination of Hamas’s leader, Ismail Haniyeh, the Middle East is on the brink of regional war. The killing occurred in Tehran after Haniyeh attended the inauguration of Iran’s President Masoud Pezeshkian, a moderate who campaigned on reengaging the West. Pezeshkian has been open to negotiating a revival of the Joint Comprehensive Plan of Action (the Iran nuclear deal). But Israel’s attack makes that less likely—and raises the odds of the U.S. getting dragged into war with...