When I heard that Hassan Nasrallah, Hezbollah’s longtime leader, had been killed last Friday in an Israeli air strike, in south Beirut, my thoughts returned to a scene that has lingered in my mind for more than a decade. In 2012, in a village called Sohmor, in Lebanon’s Bekaa Valley, I attended an open-air ceremony for two young Hezbollah fighters who had been killed in Syria, where the civil war was under way. Portraits of Nasrallah and Iran’s Supreme Leader, Ali Khamenei, were plastered on placards. Hundreds of Hezbollah fighters, wearing black uniforms, entered in formation, while a brass band played a march that could have come from a college football game. Young Hezbollah recruits, nine and ten years old—called Mahdi Scouts—served coffee to the attendees. The scene was festive, like a tailgate party before kickoff.

At the time, Hezbollah’s intervention in Syria—designed to save the teetering regime of Bashar al-Assad—was still secret. The public account put out by Hezbollah was that its two fighters had been killed in an accident in Lebanon. But secrets are hard to keep in a village. Portraits of the fallen fighters, Ali Hussein al-Khishen and Ali Mustafa Alaeddine, showed two men who looked hardly old enough to be out of high school. Their bodies had been so badly disfigured that their families had not been allowed to see them.

As the ceremony unfolded, the crowd stirred. Men appeared on rooftops, carrying violin cases with rifle butts sticking out. Hashem Safieddine, the head of Hezbollah’s executive council, walked onto the stage. He had a gray beard and wore a black turban, the latter signifying that he was a descendant of the Prophet Muhammad. Safieddine is Nasrallah’s cousin. He told the mourners, seated in rows of plastic chairs, that this was not a sad occasion but a happy one. Hezbollah, he said, was built on sacrifice. “Don’t cry, Sohmor,’’ Safieddine said. “You can’t have dignity without young blood.”

After the ceremony had finished, I introduced myself to Safieddine. He gave me a startled look, which suggested that I wasn’t supposed to be there. But what I remember most from the encounter was shaking his hand and how soft and uncalloused it was—not the hand of a man who often carried a gun.

This week, according to some press reports, Safieddine was expected to be named the new secretary-general of Hezbollah, replacing his cousin Nasrallah. During the thirty-two years that he ran Hezbollah, Nasrallah made it the world’s most powerful militia, stronger than both Lebanon’s Army and the state. Hezbollah was created by Iran, and is funded, armed, and sustained by Iran—Lebanese in name but Iranian in loyalty, with the declared purpose of destroying Israel. The easiest way to visualize Hezbollah is as an Iranian aircraft carrier sitting off Israel’s coast, from which its missiles require the briefest flight times to reach their targets. Nasrallah posed as a kind of Che Guevara of the Arab world—mocking his enemies, living mostly underground.

After Nasrallah’s death, though, the entire Middle East is changed. On Monday, Israel launched an invasion of Lebanon; on Tuesday, Iran fired nearly two hundred ballistic missiles toward Israel, potentially pushing the region toward a regional war, and one that could draw in the United States. The current conflict on the Lebanese border began last October 8th—the day after Hamas militants from Gaza massacred twelve hundred people in southern Israel and took a couple of hundred more hostage—when Hezbollah began firing rockets into the northern part of the country. As recently as last week, at the United Nations, President Joe Biden, with President Emmanuel Macron, of France, and other allies, pressured Israel to accept a ceasefire in Lebanon. The Israeli Prime Minister, Benjamin Netanyahu, ignored them and pressed the attack. In a series of air strikes throughout the past several months, culminating in Nasrallah’s killing, Hezbollah has been severely degraded, with as many of its senior leaders dead as alive, including Ibrahim Aqil, Fuad Shukr, Ali Karaki, and several others. Safieddine, judging from his speeches, appears every bit as committed to the militant cause as his cousin was. Still, if Safieddine has been given the top job, it’s hard to imagine that he accepted it with much enthusiasm.

It is too early to know how the Israeli attacks will affect Lebanon itself. More than a thousand civilians are dead, more than a million have fled their homes, and entire city blocks have been razed. Since 2005, when the Lebanese people rose up and expelled the Syrian Army, Hezbollah—Syria’s ally—has grown ever stronger in its place. The clearest illustration of Hezbollah’s power is its refusal to allow the Lebanese parliament to appoint a President, based on the refusal of most of Lebanon’s elected representatives to approve Hezbollah’s (and Iran’s) man. In a very real sense, Hezbollah holds Lebanon hostage. On Tuesday, I spoke with Chibli Mallat, an international lawyer who was previously a candidate for the Lebanese Presidency. In the past several days, Mallat has seen much of his home city, Beirut, reduced to ruins. “I feel sad and powerless,’’ he told me. “Like many Lebanese, I hate Bibi for what he is doing, but I am very angry at Hezbollah for dragging us into this.”

Hezbollah still retains a fearsome arsenal, including several hundred long-range, precision-guided missiles, capable of hitting any target in Israel. If launched in sufficient numbers—possibly in coördination with Iran—those missiles could overwhelm Israel’s anti-missile defenses, potentially kill thousands of people, and lay waste to much of the country. Before last week’s air strikes, Israeli officials believed that, in a war, Hezbollah could fire up to three thousand missiles a day for weeks on end. According to press accounts, officials now say that the strikes appear to have reduced the number of Hezbollah’s missiles and rockets by about half.

But to attack Israel in such a way now could very likely invite Hezbollah’s destruction. It may be that the group will choose to survive and to rebuild. “Whatever is left of the Hezbollah leadership, they have to be thinking that none of them is safe,’’ Reuel Marc Gerecht, a former C.I.A. Iranian-targets officer, told me. “That could induce quite a level of paralysis.”

The harder choices, it seems, lie with Israel and Iran. Israel’s decision to send its forces into southern Lebanon is very risky, even if necessary. During the past year, some sixty thousand residents of northern Israel have been forced by Hezbollah’s bombardments to evacuate their villages and kibbutzim. Netanyahu has declared that enabling them to return is a principal aim of the war. And those Israelis can’t—and won’t—go home until they can be assured that they will not become victims of Hezbollah, whose fighters are dug in just across the frontier and who maintain scores of rocket launchers there. When I visited a Hezbollah commander this spring, at his home a couple of miles from the Israeli border, I asked him if he would be willing to pull his men back in order to reassure the Israelis. He laughed and pointed to the border. “The only direction I’m going is that way,’’ he said.

Speaking to people in the Israeli government earlier this year, I found opinions on launching a ground invasion to be mixed. Eyal Hulata, a former Israeli national-security adviser and a fellow at the Foundation for Defense of Democracies (F.D.D.), told me that there was no plausible way to secure Israel’s northern border without sending troops into Lebanon. Airpower would not be enough. “There would have to be a ground invasion,’’ he said.

Still, Israel’s previous ventures into Lebanon have proved difficult and even disastrous. In 1982, Israeli troops invaded Lebanon for what was initially intended to be a temporary operation, and they stayed for eighteen years, bogged down in a quagmire. That invasion and subsequent occupation gave rise to Hezbollah, whose fighters were drawn mostly from Shiite towns in the south. “They forced the Israelis out of Lebanon under fire,” Robert Baer, a former C.I.A. operative in Lebanon, told me earlier this year. “No one had ever done that.” In 2006, when the Israelis invaded again, Hezbollah fought them to a draw, and the Israelis departed after little more than a month. A big Israeli invading force is exactly what Hezbollah has spent the past eighteen years preparing to fight.

A revelation of the 2006 war was the strength and sophistication of Hezbollah’s tunnel networks. A few years after the war ended, I obtained the G.P.S. coördinates of a bunker that the group had used. It was located at Wadi Naim, about three miles from the border, hidden from view in a valley wall. I opened a trapdoor and found a ladder that led a hundred feet down to a large bunker supported by walls of reinforced steel. Inside, black bats hung from the ceiling. There was a kitchen, a bathroom, and rows of beds—enough space for a group of Hezbollah fighters to survive for perhaps weeks, undetected by the Israelis. When I was in Lebanon earlier this year, senior Hezbollah commanders told me that the tunnel networks are now larger and stronger. “We have tunnels for cars, tunnels for trucks, tunnels for railroad cars,” one said.

Mark Dubowitz, the chief executive of the F.D.D., who speaks regularly with Israeli officials, told me that military commanders have imbibed the lessons of 2006. But, he added, wars that begin with limited aims often expand as they become more difficult. “I hope that the Israelis are mindful of the traps that they walked into in 2006,” he said.

One big question is how Iran’s leaders will decide to carry on after Tuesday’s missile attack on Israel. The attack was largely thwarted by Israel’s missile defenses, but Israel has vowed to retaliate.

Iran has spent billions of dollars in the past decade building the “Axis of Resistance”—a network of proxy armies that stretches from Lebanon and Iraq to Syria and Yemen. The crown jewel of that network is Hezbollah, and Israel has been systematically destroying it. Can the Iranians stop them? Something that the recent fighting has revealed is how Iran’s and Hezbollah’s prowess and capabilities were overestimated—and Israel’s under-regarded. In April, following Israel’s assassination of a senior Iranian general in Syria, Iran fired some three hundred missiles and drones directly into Israel—and nearly every one was intercepted or shot down.

But, if Iran can’t protect Hezbollah, the bigger question for Iran’s leaders is: can they protect themselves? For the past several years, they have viewed Hezbollah as a kind of insurance policy against an Israeli attack, especially as they developed the infrastructure necessary to build a nuclear weapon. Tehran, after all, is a thousand miles from Tel Aviv; Lebanon is just next door. Previously, if Israeli leaders had decided that they wanted to attack Iran, they would have had to expect a large barrage of missiles from Hezbollah. Such a response now appears to be in question. “There has to be enormous paranoia in the Iranian system now,’’ Karim Sadjadpour, a fellow at the Carnegie Endowment for International Peace, told me. “If they try to strike Israel, they face massive retaliation. If they do nothing, they look weak in the eyes of their proxies and their own people—that is not a good look for a dictatorship.”

In the past few years, with Hezbollah acting as a shield, Iran has inched closer to acquiring a nuclear weapon. According to a recent report by the U.S. Director of National Intelligence, the regime in Tehran had recently begun to undertake “activities that better position it to produce a nuclear device, if it chooses to do so.” Most American experts believe that Iran has enriched enough uranium to about sixty-per-cent purity to build about three nuclear weapons; if the Iranians were to decide to deploy a nuclear weapon, they would need to enrich that uranium to ninety-per-cent purity—a process that could be done in a matter of weeks. Then the regime would have to build a trigger for the device. Some U.S. experts maintain that the Iranians do not yet have that capability, but others think that they are, at least, much closer to it than they were. In a report published in August, David Albright and Sarah Burkhard, of the Institute for Science and International Security, wrote that Tehran appeared to have shortened the time it needed to achieve “breakout”—acquiring a bomb—to less than six months. “Iran can make a crude nuclear weapon far faster than commonly assessed,’’ they wrote.

That brings us back to Hezbollah: if the Iranian insurance policy in Lebanon has been diminished, Khamenei could decide that the time has come to deploy a nuclear weapon. This could give him a sense of security that recent events have stripped away. “The simplest thing for Khamenei to do is build a bomb,’’ Gerecht, the former C.I.A. officer, said.

Few of the American and Israeli officials I have spoken with believe that Israel, acting alone, could destroy Iran’s nuclear infrastructure, especially the underground nuclear-enrichment site at Fordow, south of Tehran. The only air force capable of destroying that reactor is that of the United States. Like Presidents Donald Trump and Barack Obama before him, Biden has never ruled out using military force to stop the Iranian regime from acquiring a nuclear weapon, but he has shown little willingness to contemplate such a momentous action. On Wednesday, Biden said that he would not support an Israeli attack against Iran’s nuclear facilities. That does not mean the Israelis wouldn’t attempt one, and a strike that only partially succeeded could put enormous pressure on the United States to finish the job.

Israel’s success against Hezbollah could encourage the Israelis to undertake more aggressive action against the Iranian regime, including encouraging dissent within the country or even attacking it directly. What might such a campaign look like? Possibly strikes against Iran’s leaders, or its oil-export facilities, or its military bases. In a three-minute video addressed to the Iranian people, released on Monday, Netanyahu suggested that such activities may be about to begin. “When Iran is finally free—and that moment will come a lot sooner than people think—everything will be different,’’ he said. ♦