As word spread on Saturday that Hezbollah’s leader Hassan Nasrallah had been killed in his underground Beirut bunker by an Israeli airstrike, people began quietly reckoning with the possibility that Lebanon’s political architecture might be about to shift for the first time in more than three decades. And that, in turn, raised the prospect that locked doors might soon open across the Middle East. Those who have fought against Hezbollah—not just Israelis but also Lebanese from across the nation’s confessional divides, as well as Syrians and Yemenis—could see the tantalizing possibility that the Shiite movement’s dominance might be at an end. Many others worried that a sudden power vacuum might lead Lebanon back to the kind of civil war that tortured its people for 15 years before Hezbollah emerged in the early 1980s. Nasrallah was more than a political leader. After 32 years in power, he had become synonymous with Hezbollah, the most well-armed non-state actor in the world and the linchpin of Iran’s tentacular “axis of resistance” to Israel and the United States. You …