WASHINGTON >> The commandos came in helicopters, on a nearly moonless night.

The guards outside the secret weapons lab in northwestern Syria had already been killed in a short series of airstrikes when the thrumming of Israeli air force helicopters approached.

At first, the Sept. 8 strike seemed like many others carried out by Israel, which had long had the facility in its sights. Israeli officials believed that Iran and the Lebanese militia Hezbollah planned to manufacture a new generation of precision missiles there.

But this was a different kind of operation. The helicopters flew low, without lights. Soon, dozens of camouflage-clad commandos were rappelling down cables and rushing into the complex, which in places was more than 200 feet underground, according to U.S., European and Israeli officials.

What followed was one of the most daring Israeli military operations against Hezbollah in years.

In the panoply of Israeli strikes and assassinations since the deadly Hamas-led attack on Oct. 7, the commando raid stands out.

Israel sent special operations troops, boots on the ground, into a sovereign country to carry out a mission.

It initially looked to be a one-off attack on a Hezbollah weapons facility, but it now appears to have been the opening salvo in a covert-in-name-only campaign against the Iranian proxy group.

Hezbollah began attacking northern Israel soon after Oct. 7, and the two sides have been exchanging fire ever since. Then, on Sept. 17, about a week after the raid, thousands of pagers carried by Hezbollah members exploded across Lebanon. The next day, hundreds of walkie-talkies used by the militants detonated. Israel has neither confirmed nor denied a role in the attacks, which killed dozens of people and injured more than 3,000.

Israel then launched a heavy bombing campaign that has killed hundreds of people in Lebanon — including, Hezbollah confirmed Saturday, its leader, Hassan Nasrallah.

How Hezbollah and Iran will react remains an open question. Hezbollah has yet to mount an effective counterattack, but American officials believe the group wants vengeance for the string of attacks that began with the raid on the Syrian facility.

A warning

Nasrallah vowed that Israel would “face just retribution and a bitter reckoning.”

“This retribution will come,” he said last week. “Its manner, size, how and where — these are things we will certainly keep to ourselves, in the narrowest circles even among us.”

This account of the Syria raid is based on interviews with U.S., European and Israeli officials who are familiar with aspects of the operation. The officials spoke on the condition of anonymity to discuss sensitive intelligence.

Before conducting the raid, Israel notified senior American officials, including Gen. Michael E. Kurilla, the head of U.S. Central Command, according to a U.S. official. That Sunday, Kurilla was in Israel visiting the deep-underground war room of the Israel Defense Forces’ Northern Command, where he was presented with the military’s operational plans for countering Hezbollah, according to the Israeli military.

Israeli intelligence had information about the location of the guard posts and the layout of the facility. After the guards were killed that Sunday night, the 100 or so Israeli commandos who rappelled down from the helicopters were able to enter the site unchallenged.

Using stolen maps, the commandos split into teams, each tasked with destroying a different part of the facility.

15-minute raid

The raid lasted about 15 minutes.

The factory, which is near the city of Masyaf in northwestern Syria, is officially known as the Syrian Scientific Studies and Research Center.

But while nominally controlled by the Syrian Ministry of Defense, Western officials say Hezbollah and Iran have taken over the operations.

It is only about 30 miles from the Lebanese border, which is why, Israeli and American officials say, Hezbollah manufactures weapons there instead of in Iran. Transporting missiles and weapons from Iran to Lebanon is an odyssey of more than 1,000 miles, with an array of challenges.

For years, Israel has been tracking and trying to destroy the facility and those associated with it. In 2018, Aziz Asbar, one of Syria’s most important rocket scientists, was killed in Masyaf by a car bomb thought to have been planted by the Mossad, the Israeli spy agency.

A year earlier, Israeli warplanes also struck Masyaf, killing two people, according to Syrian officials, as part of Israel’s efforts to prevent Syria and Hezbollah from acquiring advanced weapons.