Mustafa Badreddine's death will hurt the group's operational efforts and morale in Syria, but it remains to be seen who it will blame for the attack and whether it will retaliate.

On May 13, Hezbollah confirmed the death of its most prominent military figure, Mustafa Badreddine, reportedly killed in an explosion in Damascus on Tuesday night. Given Badreddine's role as head of the group's External Security Organization and its forces in Syria, his death represents Hezbollah's biggest loss since the 2008 assassination of former "chief of staff" Imad Mughniyah. The two men knew each other very well -- they were cousins and brothers-in-law, and they led Hezbollah's military activities for years.

Badreddine (aka "Zulfiqar") had a long history in the organization's ranks dating back to the early 1980s, when he took part in a series of terrorist attacks in Lebanon and Kuwait targeting U.S. embassies, Marine barracks, and other sites. After his escape from Kuwaiti prison during the Iraqi occupation in the early 1990s, he returned to Lebanon and quickly climbed up Hezbollah's ranks, helping the group establish some of its most notorious units. One fellow operative even described Badreddine as "more dangerous" than Mughniyah, his longtime "teacher in terrorism" (see PolicyWatch 1833, "Senior Hizballah Official Wanted for Murder").

In 2008, after Mughniyah was killed in an explosion in Damascus, Badreddine was promoted to head of Hezbollah's operations, including its operations abroad. Yet he remained a shadowy figure in Lebanon until 2011, when a long-delayed special tribunal named him as a culprit behind the assassination of former prime minister Rafiq Hariri. That same year, Badreddine's status as one of the group's military pillars was further solidified when he received the Syria portfolio. According to the U.S. Treasury Department, which imposed sanctions on him for various activities, his new post included attending meetings between Hezbollah leader Hassan Nasrallah and Syrian president Bashar al-Assad, coordinating the deployment of fighters next door, and planning some of their operations in the war.

Such a background earned Badreddine many enemies, raising the question of who killed him. Every time a senior Hezbollah operative dies in a mysterious way, the usual suspect is Israel, and this time is no exception -- some reports are already indicating that it might be behind the incident. True, many Israeli decisionmakers will be happy to see Badreddine out of the picture. Yet given his prominence, it is also true that they would think long and hard before deciding to take him out, since the risk of escalation grows after such a high-profile assassination, even when conducted covertly. Israeli officials do not want an escalation on the northern front at the moment. Another possibility is that Israel was targeting an arms transfer and killed Badreddine by mistake, or that it was trying to prevent an imminent attack by Hezbollah, but this is speculation for now.

Rebel groups in Syria are potential suspects as well. Badreddine's role in that country made him a high-value target for them, since Hezbollah is responsible for killing thousands of Syrians. While it is unlikely that he was killed on the front lines, rebel units may have shelled his location behind the lines.

Less likely suspects are the Gulf governments supporting the rebellion, who will be happy about Badreddine's death but have limited capability to actually execute such an operation in Damascus. Even Hezbollah itself is a potential suspect given rumors about Badreddine's poor performance, sloppiness, instability, and impetuousness in recent years. If the group's leadership did in fact want him gone and believed he would not go quietly, they may have decided that taking him out was necessary.

Regardless of who is behind the killing, the bottom line is that Badreddine's death is a significant blow to Hezbollah, operationally and mentally. The group will now need to send another high-level official to oversee operations in Syria -- someone with vast military experience and deep knowledge of the Syrian theatre. Two possible replacements are Ibrahim Aqil and Fuad Shukr, both of whom serve on Hezbollah's highest military body (the Jihadi Council) and are already involved in the Syrian theatre. The incident is also a big blow to the group's image as undefeatable and untouchable. If Badreddine can be killed in Syria, no Hezbollah commander is safe there.

Going forward, the crucial thing to look out for is who Hezbollah blames for the killing, since publicly ascribing blame would force it to retaliate. For now, the group has apparently ordered its cadres to stop speculating about the culprit, but it is no doubt investigating what happened, and Nasrallah will eventually need to blame someone, in part to show supporters in Lebanon that the group does not back down. If Hezbollah blames Israel in the end, it is safe to assume that it will retaliate, meaning that Israelis will be heading into a long period of high alert on their northern border and abroad.

Nadav Pollak is the Diane and Guilford Glazer Foundation Fellow at The Washington Institute. Matthew Levitt is the Institute's Fromer-Wexler Fellow and director of its Stein Program on Counterterrorism and Intelligence.