Israel on Tuesday began what it labelled as a “limited” ground incursion against Hezbollah in southern Lebanon.

Iran’s foreign ministry, Hezbollah’s media office and the office of Israel’s Prime Minister Benjamin Netanyahu, which oversees the country’s foreign intelligence agency Mossad, did not reply to requests for comment.

Nasrallah’s assassination followed two weeks of precise Israeli strikes that have destroyed weapons sites, eliminated half of Hezbollah’s leadership council and decimated its top military command.

Iran’s fears for the safety of Khamenei and the loss of trust, within both Hezbollah and Iran’s establishment and between them, emerged in the conversations with 10 sources for this story, who described a situation that could complicate the effective functioning of Iran’s Axis of Resistance alliance of anti-Israel irregular armed groups.

Founded with Iran’s backing the 1980s, Hezbollah has long been the most formidable member of the alliance.

The disarray is also making it hard for Hezbollah to choose a new leader, fearing the ongoing infiltration will put the successor at risk, four Lebanese sources said.

“Basically, Iran lost the biggest investment it had for the past decades,” said Magnus Ranstorp, a Hezbollah expert at the Swedish Defence University, of the deep damage caused to Hezbollah that he said diminished Iran’s capacity to strike at Israel’s borders.

“It shook Iran to the core. It shows how Iran is deeply infiltrated also: they not only killed Nasrallah, they killed Nilforoushan,” he said, who was a trusted military adviser to Khamenei.

Hezbollah’s lost military capacity and leadership cadre might push Iran towards the type of attacks against Israeli embassies and personnel abroad that it engaged in more frequently before the rise of its proxy forces, Ranstorp said.

Nasrallah’s death has prompted Iranian authorities to thoroughly investigate possible infiltrations within Iran’s own ranks, from the powerful Revolutionary Guards to senior security officials, a second senior Iranian official said. They are especially focused on those who travel abroad or have relatives living outside Iran, the first official said.

Tehran grew suspicious of certain members of the Guards who had been travelling to Lebanon, he said. Concerns were raised when one of these individuals began asking about Nasrallah’s whereabouts, particularly inquiring about how long he would remain in specific locations, the official added.

The individual has been arrested along with several others, the first official said, after alarm was raised in Iran’s intelligence circles. The suspect’s family had relocated outside Iran, the official said, without identifying the suspect or his relatives.

The second official said the assassination has spread mistrust between Tehran and Hezbollah, and within Hezbollah.

“The trust that held everything together has disappeared,” the official said.

The Supreme Leader “no longer trusts anyone,” said a third source who is close to Iran’s establishment.

Alarm bells had already rung within Tehran and Hezbollah about possible Mossad infiltrations after the killing in July of Hezbollah commander Fuad Shukr in an Israeli air strike on a secretive Beirut location while meeting an IRGC commander, two Hezbollah sources and a Lebanese security official told Reuters at the time.

That killing was followed a few hours later by the assassination of Hamas leader Haniyeh in Tehran.

Unlike Haniyeh’s death, Israel publicly claimed responsibility for the killing of Shukr, a low-profile figure who Nasrallah nonetheless described, at his funeral, as a central figure in Hezbollah’s history who had built its most important capabilities.

Shukr was key to the development of Hezbollah’s most advanced weaponry, including precision-guided missiles, and was in charge of the Shiite groups operations against Israel over the past year, Israel’s military has said.

The current Israeli escalation follows almost a year of cross-border fighting after Hezbollah began rocket attacks in support of its ally Hamas. The Palestinian group killed 1,200 people and seized 250 hostages in an attack on Israel on October 7, 2023, according to Israeli tallies.

In Gaza, Israel’s retaliation has killed at least 41,689 Palestinians, according to the Gaza health ministry.