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Fouad Shukr, the Hezbollah commander targeted in an Israeli strike on Beirut on Tuesday night, is regarded as having played a major role in the build-up of the Lebanese group's missile capabilities and its expansion into Syria in the last decade, furthering Iran's rise as a regional power.
If his death is confirmed, Shukr would be the most senior Hezbollah figure to be assassinated since Imad Mughniyeh, its planner who was killed in Damascus in February 2008.
Two former Syrian military figures described Shukr as Mughniyeh's successor in Hezbollah, and second in importance only to leader Hassan Nasrallah.
“He was crucial as an organiser, especially to Hezbollah's missile capabilities. His role in Syria lessened in recent years, but he led the Hezbollah build-up there,” one of the sources said.
The sources said Shukr was sent to Damascus soon after the start of the uprising against President Bashar Al Assad in 2011 to oversee what would become a pervasive Hezbollah presence.
There, Hajj Mohsen, as Shukr was known, was responsible for Hezbollah personnel, the group's arsenal, and setting up loyalist auxiliary brigades.
His contact on the Syrian side was Maj Gen Basam Al Hassan, the director of Presidential Palace Security and Military Office and one of the people closest to Mr Al Assad outside of his family circle, sources said.
In 2019, the US government's Rewards for Justice programme offered $5 million for information leading to Shukr's whereabouts. It described him as a senior adviser on military affairs to Mr Nasrallah.
Shukr, the US government said, also played a central role in the October 23, 1983 bombing of the US Marine Corps barracks in Beirut.
The attack, one of the most defining moments in Middle East history, killed 241 personnel and wounded 128. It showed the lethal reach of Iranian-backed Shiite militants and led to the end of most of the direct US involvement in the Lebanese civil war.
Shukr is the latest of many Hezbollah commanders and senior figures linked to the group to be targeted by Israel in Lebanon and in Syria since the war in Gaza started in October.
Israel's military said the strike on Tuesday was in retaliation for a rocket attack from Lebanon that killed 12 Druze children. Shukr, they said, was the commander responsible for the attack. Hezbollah denied any involvement in the strike.
He was also responsible for “the killing of numerous additional Israeli civilians”, the Israeli military said in a statement.
Hezbollah has attacked Israeli targets in northern Israel and in the occupied Golan Heights as part of a wider, Iranian-led retaliation against Israel for its invasion of the Gaza Strip.
Palestinian health officials say that more than 39,400 Palestinians has been killed in the Gaza war, which started on October 7, when Hamas and other militants supported by Iran killed 1,200 civilians in an attack on southern Israel.
Hezbollah has signalled in recent days that it will respond to any Israeli retaliation but so far it has not taken any significant action.