Iran has vowed ‘harsh punishment’ for Ismail Haniyeh’s killing, but how likely is all-out war with Israel?

Iran’s supreme leader, Ayatollah Ali Khamenei, has vowed to inflict “harsh punishment” on Israel in retaliation for the assassination of the top Hamas political leader, Ismail Haniyeh, in Tehran.

Haniyeh’s killing is a critical escalation in the already volatile and increasingly hostile region. But the severity of Iran’s response to Israel will depend on a number of factors. An all-out war is not a sure thing.

A strong, but calculated response

Ever since the Hamas attack on Israel on October 7 and Israel’s subsequent invasion of Gaza, Iran and Israel have demonstrated significant restraint towards one another, drawing nuanced red lines in their proxy conflict.

Neither side has wanted to escalate the tensions into a full-scale war that could engulf the entire region. Instead, they have sought to flex their muscles to avoid appearing weak, while stopping short of provoking a major conflict.

This was evident in April when an apparent Israeli strike on an Iranian diplomatic compound in Syria killed senior Iranian military commanders. Iran retaliated with a direct drone and missile strike against Israel. The attack was seemingly intentionally designed to not cause major casualities or harm to Israel. It was also telegraphed in advance, allowing Israel and its allies to shoot down the weapons.

Iran has mostly relied on its regional allies to keep the pressure on Israel as the Gaza war has continued. These allies include Hezbollah in Lebanon, the Houthis in Yemen and its proxy militant groups in Syria and Iraq.

Israel, by contrast, has followed a dual strategy: responding forcefully to Iran’s allies and confronting Iran directly on its soil in targeted strikes.

The assassination of Haniyeh will necessitate retaliation from Iran and its proxies. Khamenei tweeted that avenging his death is Iran’s “duty”.

However, it’s unlikely this act has reached the point of justifying an open war. Iranian foreign policy has adopted a high level of pragmatism, carefully calculating the military’s actions and avoiding anything that might launch a costly war with economic, political and security implications.

This doesn’t mean there won’t be a strong response. Iran’s allies, in particular Hezbollah, possess the capability of inflicting serious damage on Israel, far exceeding what they have demonstrated so far. Their attacks could intensify significantly.

A balancing act for Iran’s new government

Iran has also recently elected a new moderate government under the presidency of Masoud Pezeshkian. He has said he wants to prioritise reforming Iran’s foreign policy to create a more amicable environment in the region. This could also pave the way for better relations with Western powers, including the United States.

His administration has signalled its intention to revive the nuclear deal that then-US President Donald Trump scrapped in 2018. Iran wants relief from the international sanctions that have significantly harmed its economy and the lives of the Iranian people.

However, the assassination of Haniyeh has created a precarious situation for Iran’s government. It must balance the need for a robust response with the desire to pursue diplomatic and economic reforms. There are two important aspects to this.

First, the attack occurred in Tehran, which compels the government to issue a strong response. Pezeshkian’s tweet after the incident demonstrates this:

Iran will defend its territorial integrity and honour and make the terrorist invaders regret their cowardly action.

Any retaliation would likely increase tensions with Western countries and could hinder Pezeshkian’s diplomatic aims.

Second, Iran’s hardliners will seize this opportunity to criticise the new government, questioning its policy of reconciliation with the West. They will argue that such policies are ineffective and advocate for a more confrontational stance, thereby undermining the government’s reformist agenda.

A tactical win for Israel with potential downsides

Even though Israel has not claimed responsibility for the assassination, Prime Minister Benjamin Netanyahu’s government may view it as a tactical win. It continues to showcase Israel’s ability to target high-profile adversaries.

Israel has either confirmed it has (or is believed to have) eliminated other Hamas leaders, such as deputy political chief Saleh al-Arouri, deputy chief of military operations Marwan Issa and potentially the commander of the al-Qassam Brigades, Mohammed Deif (though his death has not been confirmed).

This week, Israel also claimed responsibility for killing a senior Hezbollah commander, Fuad Shukr, in a strike in Lebanon.

However, given the significance of Haniyeh to Hamas and the fact the killing occurred in Tehran, it comes with significant risks and potential downsides.

The assassination will almost certainly hinder the ceasefire negotiations between Hamas and Israel. This will complicate efforts to secure the release of the remaining Israeli hostages. The Israeli public is already deeply divided, with the government facing criticism for its security failures in the October 7 attack and its inability to bring the hostages home.

Cracks have also begun to emerge between Israeli military leaders and the government over the war in Gaza.

In this context, the assassination of Haniyeh is unlikely to unite Israelis. Instead, it may exacerbate existing divisions, particularly if it leads to more conflict.

Where does Hamas go from here?

Haniyeh had a significant role in Hamas’s leadership. He facilitated strategic relationships with influential regional players, including Iran, Turkey and Qatar. He also played a crucial role in mediating between different factions within Hamas, and between Hamas and other Palestinian militant groups.

This is especially important due to the recent agreement struck in China between 14 Palestinian factions, including Hamas and its main rival, Fatah. This deal aims to unite them to create a strong political entity to pursue a potential peace process with Israel.

His absence will undoubtedly affect Hamas. However, it is not the first time Hamas has lost prominent leaders; Israel assassinated Hamas founder Ahmed Yassin in 2004, among others.

Despite these losses, Hamas has maintained strong leadership and regional backing, particularly from Iran, Turkey and Qatar.

It is still too early to determine who will replace Haniyeh, but his successor will need to possess a similar level of regional connections and public support.

A likely candidate could be someone like Khaled Mashal, who has previously held leadership roles within Hamas and has substantial regional influence.

Ali Mamouri does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.