Lebanon’s Hezbollah militia announced Friday that its top military commander in the Syrian civil war died in a mysterious blast in Damascus, dealing a major blow to the powerful Iranian-backed group.

The killing of Mustafa Badreddine, 55, comes as Hezbollah struggles to balance combatting its traditional nemesis, Israel, with its costly intervention in the Syrian conflict to bolster President Bashar al-Assad’s forces against rebel factions, including some groups backed by the United States and its allies.

Badreddine is the most senior Hezbollah official killed in Syria. He was linked to deadly attacks in 1983 on U.S. and French embassies in Kuwait, and was among four people indicted by a U.N. tribunal for involvement in the 2005 assassination of Lebanese Prime Minister Rafiq al-Hariri. […]