In the turbulent landscape of the Middle East, Iran’s aging supreme leader, Ayatollah Ali Khamenei, could always rely on the close alliance, unwavering loyalty and deep friendship of Hassan Nasrallah, the leader of the Lebanese militia Hezbollah.

When Israel killed Mr. Nasrallah in a massive airstrike on Friday, it abruptly wiped out a singular force in Mr. Khamenei’s hierarchy of close associates.

Iran had for 40 years nurtured Hezbollah as the main arm of its proxy network of militias, as a forward defense against Israel. But in the past two weeks, Hezbollah’s capacity began to crumble under wave after wave of Israeli attacks on its leadership, arsenal and communications.

Now, fissures have opened within the Iranian government over how to respond to Mr. Nasrallah’s killing, with conservatives arguing for a forceful response and the moderates, led by Iran’s new president, Masoud Pezeshkian, calling for restraint.

All of this has left Iran, and its supreme leader, in a vulnerable position.

Four Iranian officials who knew Mr. Nasrallah personally and had been briefed on events said that Mr. Khamenei had been deeply shaken by his friend’s death and was in mourning, but had assumed a calm and pragmatic posture. The officials, including two members of the Islamic Revolutionary Guard Corps, requested anonymity as they were not authorized to speak publicly.

Mr. Khamenei struck much the same tone in public. Instead of lashing out at Israel, he issued two restrained statements, praising Mr. Nasrallah as a leading figure in the Muslim world and the so-called axis of resistance, and saying that Iran would stand by Hezbollah.