By Rebecca Armitage, Toby Mann and Basel Hindeleh, ABC News
Iranian mourners are holding portraits of Hamas leader Ismail Haniyeh during a funeral ceremony for him and his bodyguard Abu Shaaban in Tehran, Iran, on August 1, 2024. Photo: Morteza Nikoubazl / NurPhoto via AFP
Iran says it has a "duty" to avenge the assassination of Hamas leader Ismail Haniyeh, who was killed during a visit to Tehran this week.
Describing Haniyeh as a "dear guest", the country's Supreme Leader Ayatollah Ali Khamenei has placed the blame for Wednesday's assassination squarely on Israel.
"Following this bitter, tragic event which has taken place within the borders of the Islamic Republic, it is our duty to take revenge," his account wrote on X.
While Israeli authorities have neither confirmed nor denied their involvement in the plot that appears to have involved an "airborne projectile", the country said it is prepared for "challenging days ahead".
But what, exactly, might Iran do next?
Regional tensions have been simmering since Hamas-led militants streamed across the border from Gaza into southern Israel in October last year, killing hundreds of civilians and taking more than 250 hostages.
And while Israel and the Iran-led Axis of Resistance have engaged in an increasingly risky tit-for-tat ever since, neither side has so far expressed a desire for war.
Haniyeh's death, however, may change calculations on both sides.
Women wave flags as they protest near the Israeli embassy in Amman, to denounce the assassination of Palestinian Hamas chief Ismail Haniyeh, on August 1, 2024. Photo: AFP
Middle East experts say an attack in the very heart of Iran has humiliated the country's leaders.
"Haniyeh's death mere hours after the inauguration of the new Iranian president on Iranian soil is a huge, humiliating blow to the Islamic Republic's prowess and prestige," said Ahmed Fouad Alkhatib, from the US think tank the Atlantic Council.
"There's no way that the [Islamic Revolutionary Guard Corps] will not respond directly or in a dramatic fashion."
From the drastic to the restrained, these are the potential responses Iran might be considering.
A drone strike
The country has to do something, according to unidentified Iranian officials who spoke to the New York Times.
One scenario, the officials said, would see them use a combination of drones and missiles that take care to avoid civilian targets.
Such a strategy would be similar to one Iran used against Israel in April.
Back then, an Israeli attack on a diplomatic facility in Damascus killed seven Islamic Revolutionary Guard Corps (IRGC) officers, prompting Iran to retaliate with a barrage of more than 300 missiles and drones.
It was the Islamic Republic's first direct attack on the Jewish state, bringing out of the shadows a decades-long cold war between these two enemies.
But Israel's sophisticated Iron Dome defence system intercepted "99 per cent" of the projectiles, according to military officials.
At the time, US President Joe Biden reportedly told Israeli Prime Minister Benjamin Netanyahu the Iron Dome had prevailed - with a little help from Western allies - and implored him to "take the win".
But the events of this week might be more personal for Iran and its allies.
Israel took responsibility for the assassination of top Hezbollah commander Fuad Shukr in an air strike on Beirut.
Then hours later, Haniyeh was killed.
While Israel's involvement is unconfirmed, Iran has long maintained that spies from its Mossad agency are operating in their territory.
"Authorities of the Islamic Republic should be concerned about their lives; the Mossad has infiltrated various parts of the country," Iran's former Minister of Intelligence, Ali Younesi said in 2021.
Haniyeh's death will rattle Iran's leadership, experts claim.
"The strikes likely eliminated a false sense of security for leaders in the Iran threat network of proxy forces and were also meant to re-establish deterrence against attacking Israel," said the Atlantic Council's Alex Plitsas.
"While the April strike was more symbolic and meant to convey a deterrence message, the strikes in the last two days decapitated senior leaders in the threat network attacking Israel."
A response by proxy
Iran has a network of regional allies known as the Axis of Resistance.
It includes Hezbollah, which has been trading fire with Israel in southern Lebanon.
Tehran has also backed Shiite militias across the region, notably in Iraq, Syria and the Houthi rebels who control much of Yemen.
The Houthi rebels have already targeted enough ships to provoke the US and UK into launching air strikes against them, and sending their navies into the area to thwart further attacks.
Using these forces to retaliate could give Iran the ability to respond indirectly, but it's unclear how effective they would be.
If Iran was to unleash Hezbollah, it could become a far greater foe for Israel to face than Hamas.
Hezbollah, described as the most-armed militant group globally, has said it could strike anywhere in Israel with its rockets.
It also has a store of more sophisticated weapons, and is believed to have up to 150,000 missiles near Lebanon's border with Israel.
Hezbollah leader Hassan Nasrallah said the militia has 100,000 fighters, but the CIA World Factbook estimated there were about 45,000 fighters in its ranks in 2021.
Many are battle-hardened veterans of the Syrian civil war.
Hezbollah may not need too much encouragement to retaliate after the death of Fuad Shukr, who was assassinated on Lebanese soil.
Whether a reprisal is launched directly by Iran or via one of its allies, Israel said it was ready to respond forcefully to any retaliatory attack.
"We are prepared for any scenario and we will stand united and determined against any threat. Israel will exact a heavy price for any aggression against us from any arena," Prime Minister Benjamin Netanyahu said in a televised address after Haniyeh's assassination.
If an attack was launched by Iran and its proxies, experts say Israel could struggle to defend itself.
"The Islamic Revolutionary Guards Corps similarly published a statement saying that the Axis of Resistance and 'especially Iran' will retaliate against Israel," the Institute for the Study of War said.
"Such a combined Iranian-Axis of Resistance attack would be especially dangerous because attacking from multiple directions would put greater pressure on Israeli air defences than the Iranian attack in April 2024 did."
All-out regional war
The big fear with tensions rising and Iran being internationally humiliated by Haniyeh's killing is that it will be pressed to marshal its forces for an all-out war.
Regional conflict could drag in allies and proxies from both sides, and things would escalate quickly.
William Wechsler, senior director of Middle East Programs at the Atlantic Council, does not think Iran will go down this path.
"I suspect Tehran will also respond in a manner that it believes will avoid a regional war," he said.
"The primary danger today therefore remains more a question of miscalculation than of intent."
The US has raised concerns about the risks facing the region.
"They don't make the task of de-escalation, deterrence and dissuasion, which is the goal, any less complicated," White House national security spokesperson John Kirby said.
Israeli Foreign Minister Israel Katz said it was "not interested in an all-out war".
But he went on to say the only way to prevent it was by implementing a UN resolution that bans armed groups in southern Lebanon from being near the border with Israel.
"We will harm with great force whoever harms us," he wrote in a letter to other foreign minsters around the world, sent after the killing of Shukr in Beirut.
Ayatollah Ali Khamenei has warned Israel it was in line for "harsh punishment" and said it was Tehran's duty to avenge Haniyeh's death.
What form that will take is unclear, but judging by from the rhetoric from both sides, something significant is expected.
- ABC