This is a guest post by Anh Le. Anh currently works in the NGO sector on environmental issues but previously taught at the University of Manchester, where he also got his PhD, writing on the ethics of force short of war.

It’s important to note at the outset that what unfolded on Saturday October 7th in Southern Israel when Hamas fighters overran the Israel – Gaza border, infiltrated deep into Israeli territory, murdered more than a thousand Israelis, and took more than a hundred hostages back across Gaza was a war crime (or at least most of it was, the killing of Israeli soldiers, even if most of them were unarmed can be argued to be the legitimate targeting of combatants in an armed conflict). Equally important to note is how the Israel Defence Force (IDF) has responded to the initial attack also violates the International Humanitarian Law, e.g. the blockade of Gaza, indiscriminate bombings of residential areas. At the time of writing, the IDF hasn’t officially conducted a land invasion of Gaza, although some ground incursions have occurred. In this post, I argue that, contrary to what has been taken as a fact – that Israel has the right to go to war against Hamas following its attack on Israel and the only question that is morally, and legally, relevant is how they go about doing that, a question of jus in bello – it’s not clear if Israel’s war meets the criteria of jus ad bellum – the right to go war, and thus if Israel has a right to go to war against Hamas.

I should first make clear that I will not weigh in on the ethics of the situation between Israel and Palestine. The history is protracted and there are others eminently more qualified to unpack it than myself.

Let us begin with Prime Minister Netanyahu’s declaration of war following the terrorist attack on October 7th. Most world leaders, including the U.S. President, UK Prime Minister,  President of the European Commission, took this as a matter of fact. This was, prima facie, understandable. Israel, like any other states, has a right to self-defence and the declaration of war against Hamas seemed to be a correct extension and application of that. But to engage in a war of self-defence, there must be a threat that is present, ongoing, or imminent. It is not clear that the war conducted in the current manner is in line with this self-defence justification. Although the full detail of Hamas’s attack is still emerging, the best account we have seems to be this: Hamas forces stormed the Israel – Gaza border on Saturday the 7th, murdering civilians in Southern Israel, took hostages back to Gaza while a small contingent remained, or trapped, in Israel and continued fighting for several days until they were eliminated. If that’s the true account, then Israel could use military force to repel the initial attack by Hamas in its homeland, but, crucially, only up until the moment the threat is eliminated. That means that once all Hamas fighters were eliminated within Israel, the self-defence justification was no longer in effect.

The pushback against this would be: despite the elimination of Hamas fighters within Israel, Hamas poses an ongoing threat for as long as it exists. This is demonstrated through the firing of rockets towards Israel and the evidence that they had orchestrated the October 7th attack which points to possible future attacks. So the war against Hamas is still justified on the basis of defence against the ongoing threat of Hamas’s missile strikes, and the future threat of Hamas’s ground attacks.

First, with respect to the launching of rockets from Gaza towards Israel, this warrants a military response to eliminate this threat. But it is not clear how a war, conducted in the form of bombardment of the entirety of Gaza can be proportionate to meet this stated objective. It is important to note here that this is not an issue of in bello proportionality (i.e. individual casualties resulted from individual bombing) but one of ad bellum proportionality (I.e. the casualties resulted from the air-war against the benefits to be fulfilled by the achievement of just cause). The response of war has to meet the ad bellum proportionality test before one of in bello, and the foreseeable destruction of Gaza and the monumental losses of innocent civilians’ lives when one of the most densely populated enclaves in the world is subject to intense bombardment cannot satisfy this proportionality calculation.

Second, and relatedly, perhaps the elimination of rocket-launching capabilities of Hamas cannot justify a war against Hamas (after all, most of their rockets are intercepted by the Iron Dome system) but the destruction of Hamas’s capabilities to launch future ground attacks into Israel can be a reason to go to war. But as orthodox just war theory doesn’t recognise preventive war. For the war to be just, the threat has to be ongoing or is imminently about to take place. Given that no such evidence is available, it’s hard to see what, exactly, could justify a war-like response against Hamas.

Does this mean that Israel cannot do anything, militarily speaking, against Hamas? Of course not.

Israel has a right to self-defence against any threats but this doesn’t always translate to the right to respond to any threat with war. There are a range of military actions short of war that Israel can take, and indeed has taken over the course of its long and protracted history with Palestine. The use of special operations raids, limited, selected, and localised airstrikes, and so on. In addition, using force on a smaller scale avoids the risks of accidentally killing the hostages currently under Hamas captivity and prevents the risks of the conflict spilling out of control. At the time of writing, following the destruction of a hospital in Gaza with, reportedly, more than 500 civilians killed (the responsible party is yet to be determined, but perhaps that’s besides the point here), an escalation of the conflict beyond the Israel – Hamas dimension is a distinct possibility.

Following Hamas’s terrorist attack against Israel, it was clear that Israel had a right to respond. But that right to respond doesn’t automatically translate to the right to go to war. The scale of losses of innocent lives in Israel doesn’t determine the scale of military response to which Israel can justifiably turn. Equating the two is to conflate the principle of proportionality with that of revenge which cannot be morally justified.