• Israel’s assassination of Hezbollah leader Hassan Nasrallah was a major escalation.
• Nasrallah was one of Iran’s most important allies.
• Iran’s response could de-escalate the conflict or spark a wider war involving major global powers.

Israel’s assassination of Hassan Nasrallah, the head of Hezbollah for over 30 years, is a major escalation in the Middle East conflict.

De-escalation is now up to Iran.

Nasrallah was Iran’s most trusted proxy in its conflict with Israel and its power struggle with Saudi Arabia and the United States. A senior Iranian commander working with Nasrallah was also killed in the attack, Iranian state media confirmed on Saturday.

Israel last assassinated an Iranian commander in April when it struck an Iranian consulate building in Damascus, the capital of Syria, killing multiple military officers, including Brig. Gen. Mohammad Reza Zahedi, a senior commander in the Iranian Revolutionary Guard Corps’ Quds Force.

In the days after that attack, Iran telegraphed its response before launching hundreds of missiles and drones toward Israel in an unprecedented retaliation. Israel, with the help of the United States, intercepted almost all of those missiles.

Iran likely anticipated that those missiles would be largely intercepted, experts said at the time, indicating its desire to avoid a wider war with Israel, which could lead to conflict with the United States.

But as Israel shows no signs of letting up in its campaign to dismantle Hezbollah in Lebanon and Hamas in Gaza, the whole world is now waiting to see how Iran might respond this time — and whether a wider war involving major powers is on the horizon.

“The expectation might be that it is the ultimate moment for Iran to step up or step aside,” Randa Slim, a senior fellow and director of conflict resolution at the Middle East Institute, a nonprofit think tank based in Washington, DC, told Business Insider. “Iran huffs and puffs, but it is, in fact, powerless. This helplessness is due to a simple fact: Iran does not want war with Israel, which is the same as war with the United States, which likely would topple the Islamic Republic.”

Iran’s supreme leader, Ayatollah Ali Khamenei, in a statement released on Saturday, said Iran stands with Hezbollah but did not threaten any military response.

Still, Israel, anticipating a response from Hezbollah and other proxies in the region, and possibly Iran as well, ordered its citizens to avoid large gatherings. Air raid sirens rang continuously in central Israel throughout Saturday evening local time.

The Israel Defense Forces on Saturday said it intercepted a rocket fired from Yemen by the Houthis, an Iran proxy that has achieved new levels of notoriety by attacking shipping lanes in the Red Sea and launching drones and rockets toward Israel as it calls for a cease-fire in Gaza.

“It’s a precarious position for Tehran,” Jonathan Panikoff, a senior fellow at the Atlantic Council, told Business Insider. “To not do so risks undermining its legitimacy in the eyes of its proxies throughout the region and for the revolution’s supporters in Iran. But partaking in a broad, lethal response would trigger a regional war that, after April, Tehran probably recognizes it could leverage to inflict damage on Israel, but almost certainly lacks the military capability to come out of the conflict better than it went into it.”