In recent years, a number of states have claimed to be acting in self-defence when conducting an act of aggression against another state. Russia invaded Ukraine in 2022 and said it was acting in collective self-defence of the Donetsk and Luhansk republics. Israel invoked its right to self-defence in response to Hamas’ attacks on October 7 in its military operation in the Gaza Strip in 2023. India struck Pakistan in 2025 and claimed it was acting in self-defence against an armed attack by proxies.

Finally, Israel attacked Iran in 2025 and claimed it was acting pre-emptively in self-defence against its nuclear programme. Ukraine, Pakistan, and Iran each also claimed their right to self-defence against these attacks, though of course there cannot be a right to self-defence against self-defence. So what then constitutes self-defence?

The United Nations Charter

After World War II, one of the main aims of the drafters of the United Nations Charter was to seek to restrain the use of force and avoid the horrors of that war. The first purpose of the UN listed in Article 1(1) of the Charter was to maintain international peace and security. To that end, it sought to establish a general and comprehensive prohibition on the use of force and also aimed to set up an organisation (the Security Council) that would be able to take collective action to deal with threats to the peace or breaches of the peace. It also maintained each individual state’s right to self-defence as this right is an inherent prerogative of states against aggression and is a key attribute of statehood in the international legal system.

There are three key provisions which deal with the use of force in the UN Charter:

1. The prohibition on the use of force in Article 2(4) to which there are two exceptions: a. Article 51 acknowledges the inherent rights of states to individual and collective self-defence if an armed attack occurs, and; b. Article 42 allows the Security Council to authorise the use of force when the Council considers that there has been a threat to the peace, breach of the peace or act of aggression.

There are many controversial aspects to the law relating to self-defence. These include the right to anticipatory self-defence against an armed attack which has not yet occurred, the right to self-defence against attacks by non-state actors, the concept of humanitarian intervention, and finally, whether there is a right to self-defence in occupied territory.

Anticipatory Self-Defence

The concept of anticipatory self-defence concerns whether a state is allowed to use force to respond to an armed attack which has not yet occurred. This is usually divided into two distinct types - pre-emptive self-defence against armed attacks which are imminent or preventive self-defence against threats which have not yet materialised. While some states believe that preemptive self-defence is allowed, most believe that preventive self-defence is unlawful. The most quoted example for pre-emptive self-defence is Israel’s strike on Iraq’s Osirak nuclear reactor in 1981. Israel argued in its report to the Security Council at the time that the reactor was designed to produce atomic bombs, the target for which would have been Israel, and therefore it was exercising its inherent right of self-defence. However, UN Security Council Resolution 487 (1981) condemned Israel’s attack as inconsistent with the UN charter and recognised the right of States to develop their economy and industry for peaceful purposes.

Israel’s strike against Iran’s nuclear programme in 2025 rests on largely the same argument, that Iran may acquire nuclear weapons which it may use to target Israel in the future. Israel is unable to fulfil the imminent criteria in arguing that this could plausibly be a case of pre-emptive self-defence, as it cannot argue that Iran was about to attack Israel with nuclear weapons it does not have. While this argument may have gained currency with more states than their attack in 1981, by and large most states have viewed this as unlawful and an act of aggression. Russia similarly used this argument against Ukraine in claiming collective self-defence against attacks which may occur against Russia at some future point. According to most of the international community, these claims do not fulfil the criteria for self-defence as defined in the UN Charter.

Attacks by Non-State Actors

Jurisprudence by the International Court of Justice also holds that while states are not allowed to use force against each other (except for in self-defence or under Security Council authorisation), they also cannot use this force indirectly through proxies. In the Nicaragua judgment, the Court ruled that the provision of weapons and logistical support to a non-state armed group would amount to a use of force. Moreover, organising and sending the armed group to conduct an armed attack against another state would give the victim state the right to use force in self-defence against the state sending the armed group under Article 51. International law does not allow for a state to use force against another state through the use of proxies. However, the test for attribution is high, it requires ‘sending’ or ‘substantial involvement’ in an armed group does not include mere funding, arming, or giving military logistical support.

However, a question faced by many states is this; what if non-state actors conduct attacks on a state’s territory from the territory of another state without that state having sent them or even been involved with them in any way? The most used example for this is ISIS in Syria in 2014 as ISIS was conducting attacks against other states from Syria’s territory but instead of Syria sending or organising them or even harbouring them, Syria was actively involved in fighting against ISIS. In order to respond to this, states such as the US have created a very controversial doctrine called ‘unable or unwilling’ which provides that if a state is unable or unwilling to deal with a threat by a non-state actor, victim states of the group’s armed attacks can use force in that state’s territory against the non-state actor. While there is some state practice to support this, it remains a controversial doctrine which has not yet attained legal status. The test for targeting a non-state actor in another state’s territory remains that of ‘sending or organising’ as held in Nicaragua. As a result, India’s use of force against Pakistan in 2025 is an unlawful act of aggression, as it had no evidence to indicate that the armed attack in Pulwama was by an armed group sent or organised by Pakistan.

Humanitarian Intervention

This is a very controversial doctrine which arguably allows States to use force beyond the two exceptions in the UN Charter (self-defence or Security Council authorisation). Some States contend that, when the Security Council is paralysed, they recover a broad right to self-help and can use force if there is a major human rights threat in order to protect large numbers of civilians. They further argue that this is not a violation of Article 2(4) as it is consistent with the purposes of the UN which exists to promote human rights and protect succeeding generations from the scourge of war. This has been invoked by NATO in its military intervention in Kosovo in the 1990s. However, it has not gained acceptance among most states in the world and therefore, the use of force for humanitarian intervention remains illegal. However, it can be argued that any claim in favour of this exception is dead as no state is currently arguing for it in Gaza, and there could be no better case.

Right to Self-Defence in Occupied Territory?

Israel has invoked its right to self-defence against Hamas’ attacks on October 7. However, it is not clear whether a state has this right in territory that it occupies. The International Court of Justice in its 2004 Advisory Opinion notes that Israel is an occupying power in Palestinian territory and therefore the right to self-defence cannot be exercised where the threat originates from within that territory. While Israel can use force to deal with threats against it, that would not be the right to self-defence as found under the UN Charter.

Future of the Law Against War

The right to self-defence remains controversial with some states advocating for narrow exceptions to the prohibition on the use of force and others arguing for very wide ones. These broad exceptions would allow the right to anticipatory self-defence against imminent armed attacks and the right to use force against states unable or unwilling to act against non-state actors operating on their territory. State practice will decide whether the law against war can retain its integrity with states attempting to carve out these large exceptions.

This article by Ayesha Malik was produced with the support of the International Committee of the Red Cross (ICRC) as part of the Legally Speaking podcast series. The views expressed are the author’s own.