Hassan Nasrallah, the longtime leader of the powerful terrorist group Hezbollah, was killed by an Israeli airstrike Friday in Beirut in a major blow to the Lebanon-based organization and its chief patron, Iran.

Both Hezbollah and the Israeli Defense Forces on Saturday morning confirmed the death of Mr. Nasrallah. His death comes at a moment of rapid escalation between Israel and Hezbollah, and amid fears that a full-scale ground war between the two sides is imminent.

An official statement from Hezbollah said that Mr. Nasrallah, 64, “has joined his fellow martyrs” following the Israeli strike in the Lebanese capital. Lebanese officials said at least six people were killed and another 91 injured in the attack. In its statement, Hezbollah also vowed to “continue the holy war against the enemy and in support of Palestine.”

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Separately, the IDF said the Hezbollah leader had been killed.

“Hassan Nasrallah will no longer be able to terrorize the world,” the IDF said in a social media post Saturday morning.

Mr. Nasrallah is the most powerful and influential figure killed by Israel so far in its war against Hamas, Hezbollah and other arms of Iran’s “axis of resistance,” which also includes the Yemen-based Houthi rebels and Shiite militias operating in Iraq and Syria.

Iran also seemed to confirm the death of Mr. Nasrallah early Saturday. In a televised message Saturday, Iranian Supreme Leader Ayatollah Ali Khamenei called on “all regional resistance forces … to stand by and support Hezbollah.”

Israeli military officials said they had been tracking Mr. Nasrallah’s movements for years. The location and subsequent killing of the Hezbollah leader is a major success for the Israeli military and the country’s intelligence services, and it comes on the heels of a string of high-profile operations over the past several months.

Israel has targeted all of Iran’s proxy groups across the Middle East since Oct. 7, when Hamas launched a major terrorist attack against Israel that resulted in the death of more than 1,200 Israelis and the taking of more than 250 hostages, roughly 100 of whom are still believed to be held by Hamas in the Gaza Strip.

In July, Israel is believed to have conducted a strike in Tehran that killed Hamas political leader Ismail Hanieyh.

Two weeks ago, Israel is believed to have conducted a major operation targeting hundreds of Hezbollah fighters by detonating explosives hidden inside pagers and walkie-talkies used by the group. That operation dealt a serious blow to the group in a physical sense, but it also had a significant psychological effect by demonstrating the capabilities and reach of the Israeli military and intelligence 
services.

Israel has been waging war against Hamas for nearly a year, but analysts have long warned that a potential Israel-Hezbollah war could be far more deadly and disruptive to the region. Hezbollah has claimed to have as many as 100,000 well-trained fighters, though most analysts believe the true number to be around 20,000, with thousands more reservists. The group also has considerably more rockets and other weapons than Hamas, and is far more capable as a legitimate fighting force than any other Iranian proxy group in the region.

Over the past few weeks, the two sides have traded strikes and inched closer to all-out war. Hezbollah has targeted cities across Israel, including Tel Aviv, with rocket attacks, while the IDF has responded with an increasingly aggressive air campaign in southern Lebanon.

Late Friday night, hours after the strike that killed Mr. Nasrallah, the IDF said its campaign against Hezbollah will continue. It warned residents in Beirut neighborhoods, where Hezbollah has embedded its operations that more strikes are coming.

“The IDF calls on the residents of neighborhoods in the Dahieh of Beirut to move away from Hezbollah assets and facilities,” the Israeli military said in a post on X. “We will continue operating to precisely dismantle Hezbollah’s offensive capabilities. Hezbollah has strategically embedded weapons in civilian areas, putting Lebanese civilians at risk in order to harm Israeli civilians. Our war is with Hezbollah, not the people of Lebanon.”

A key question is how Iran will respond to the death of Mr. Nasrallah. In April, Iran launched a major drone and missile attack against Israel after the IDF killed Iranian military officials visiting Syria.

Nearly all of those drones and missiles were shot down by Israeli and U.S. air defenses.

The U.S. feared that Iran may again attack Israel after the death of Haniyeh, the former Hamas political leader, in July. But so far, Iran has not attacked Israel directly, and some analysts believe it’s likely to stay that way.

“It’s clear that Iran has little interest in escalating tensions with Israel and its allies right now,” former Israeli intelligence official Avi Melamed said in comments circulated to reporters Saturday. “Instead, it may allow its weakened deterrence strategy to fade while prioritizing self-preservation. In the coming days, Iran is likely to rely on its remaining proxies to ’respond’ to Israeli actions, a strategy that Israel and its allies will continue to counter.”

For the U.S., the death of Mr. Nasrallah further fuels fears that escalation in the Middle East is now inevitable or has already arrived. Earlier this week, President Biden pleaded with both sides to step back from the brink of all-out war that could devastate Lebanon and northern Israel.

“Full-scale war is not in anyone’s interest,” the president said during a speech to the United Nations General Assembly in New York City. “Even as the situation has escalated, a diplomatic solution is still possible. In fact, it remains the only path to lasting security to allow residents from both countries to return to their homes on the border safely.”