Following our earlier briefing paper1 on the chemical attack in Syria, we turn now to the possible legal, rather than moral, justification for the US military action in Syria last week. The US Administration has remained resolutely silent in setting out the legal basis of its action, save to say that it was a ‘measured response’2 to the use of chemical weapons by the Syrian government. Although the US action has enjoyed the support of its allies, with the strike being described as ‘wholly appropriate’3, no-one has agreed, or put forward, the legal basis.

It is perhaps helpful to start with the general prohibition on the use of force by a State against another UN Member State, contained in Article 2(4) of the UN Charter, which provides as follows:

‘All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in the manner inconsistent with the purposes of the United Nations’

However, the general prohibition is subject to Article 2(7)4, which permits the use of force where such action is authorised by the Security Council under its Chapter VII powers. In the present instance, there was no authorisation and, in any event, an attempt to secure such a mandate would undoubtedly have been met by a veto from Russia. This, then, leaves two other possible bases: individual or collective self-defence under Article 51 of the UN Charter; or, alternatively, the more controversial, humanitarian intervention.

Collective self-defence

Article 51 provides that:

‘Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.’

If the US is placing reliance on collective self-defence, one of the first criteria to be satisfied is an ‘armed attack …..against a Member of the United Nations’; however grave and serious the attack by the Syrian government was, it cannot be construed as an ‘armed attack’ against the US, thereby excluding the prospect of justifying military action on the grounds of ‘individual self-defence’.

If the action cannot be supported on the basis of ‘individual self-defence’, can we then place reliance on ‘collective self-defence’? Article 51 preserves the right of a State to act (arguably under customary law) in self-defence until such time as the Security Council adopts measures to ‘maintain international peace and security’. If reliance is being placed on customary law, US Administration would need to have satisfied itself that the criteria for collective self-defence (set out by the ICJ in the Nicaragua5 case) are met, namely:

  1. Prior declaration by the State concerned that it has been the victim of an armed attack; and
  2. Request by that victim State for assistance.

Neither has occurred in the present case. The only two recent examples of military action based on collective self-defence are: (i) Kuwait 1990, when the Kuwaiti government in exile sought assistance, following the invasion of Kuwait by Iraq (although this was subsequently authorised by UNSCR 678) and (ii) the military action taken by the US (and its allies) against Afghanistan following the events of 11 September.

Humanitarian Intervention

Intervention in the internal affairs of a State has long been prohibited both under customary international law and the UN Charter. However, there has been some debate as to whether a third state may intervene militarily in a State where there are concerns of serious human rights abuses against its own citizens, for instance, the reported chemical attack on civilians in Syria.

The doctrine of humanitarian intervention is certainly not free of controversy, particularly when the threat or use of force by a third intervening State is predicated on the prevailing domestic situation of a State, which is protected (even in extremis, such as civil war) under the notion of ‘territorial integrity or political independence’ in Article 2(4) of the Charter.

At the same time, there is support by many in the international community for the legitimacy of intervention, under customary law, when oppression by a State of its citizens is of such magnitude and gravity that it warrants a military response and it is the only way to ‘avert an overwhelming humanitarian catastrophe’6.

Examples of military action based on humanitarian intervention include: The ‘humanitarian corridors’ created in north and south Iraq through ‘no fly zones’ to protect the Kurdish and Shia communities; and the Kosovo crisis of 1999. In the case of Kosovo, the gravity of the circumstances prompted NATO to conduct a bombing campaign without seeking to secure UN Security Council authorisation and it remains a stark example of support for the doctrine. Indeed, the then UK Secretary of State for Defence, George Robertson, expressed his views in the following terms: ‘In international law, in exceptional circumstances and to avoid a humanitarian catastrophe, military action can be taken and it is on that legal basis that military action was taken’ and that justification was echoed in the House of Commons by the then Prime Minister, Tony Blair.

Although ‘humanitarian intervention’ has not been accepted as a settled doctrine, it may be said that, by 1999, there was a measure of consensus that it was capable of being invoked as a legal basis for military action. That having been said, it has not subsequently been a catalyst for intervention in other humanitarian catastrophes around the world, including, of course, Syria. Whilst the use of chemical weapons in Syria has not gone unnoticed, or unreported, since at least 20127, it is perhaps been remarkable that acceptance of the doctrine has not resulted in military action, at least until now. Indeed, John Kerry, as US Secretary of State, pointedly chose not to rely on humanitarian intervention in August 2013 when President Obama had determined that there should be a military response (subsequently put on hold) to the use of chemical weapons in Eastern Damascus.

As a note of caution, however, humanitarian intervention should not be looked at in isolation, but must also be viewed within the context of the ‘Responsibility to Protect’ (R2P), the global political commitment by all members of the UN that was announced at the 2005 World Summit and is intended to prevent international crimes (including ethnic cleansing). Notwithstanding the formal endorsement by Member States there is growing, but not unanimous, support for R2P. Importantly, the 2005 outcome document includes the possibility of military action:

‘In this context, we [Member States] are prepared to take collective action, in a timely and decisive manner, through the Security Council, in accordance with the Charter, including Chapter VII, on a case-by-case basis and in cooperation with relevant regional organizations as appropriate, should peaceful means be inadequate and national authorities are manifestly failing to protect their populations from genocide, war crimes, ethnic cleansing and crimes against humanity.’

It will be noted that the outcome document is envisaging, under R2P, collective action under a Security Council mandate, rather than a single third State or coalition of States initiating a military response. It is, then, perhaps unsurprising that confusion has sometimes arisen as to the relationship between R2P and humanitarian intervention. For the sake of clarity, it should be had in mind that R2P consists of three ‘pillars’, only the last of which contemplates the use of force:

‘(i) Every state has the Responsibility to Protect its populations from four mass atrocity crimes: genocide, war crimes, crimes against humanity and ethnic cleansing. (ii) The wider international community has the responsibility to encourage/assist individual states in meeting that responsibility. (iii) If a state is manifestly failing to protect its populations, the international community must be prepared to take appropriate collective action, in a timely and decisive manner and in accordance with the UN Charter.’

It is the case, then, that R2P may be distinguished from humanitarian intervention as follows:

  1. Humanitarian intervention is concerned with a military response and foresees that taking place even in the absence of a Security Council mandate, but R2P is primarily a preventive initiative albeit with the prospect of military action as a last resort and subject to Security Council mandate.
  2. R2P was intended to be an expression of accepted international law, not a new or distinct doctrine. Conversely, humanitarian intervention is generally accepted to be a customary law doctrine that has evolved in relatively recent history.
  3. The focus of R2P is on the international crimes of genocide, war crimes, ethnic cleansing (although not in itself a crime defined under international law) and crimes against humanity. However, humanitarian intervention is not so confined.
  4. The jurisprudential basis for humanitarian intervention is that there is a ‘right’ to intervene", whereas R2P focuses on a series of ‘responsibilities’. (That having been said, however, both R2P and humanitarian intervention carry an acceptance that sovereignty is not, in every circumstance, absolute.)

Finally, a word must be said about the notion of ‘reprisal’; in other words, a limited and deliberate violation of international law by the use of military force in order to punish a State for its breach of international law. Given the provisions of Article 2(4) of the UN Charter (see the discussion, above), a reprisal (as opposed to a countermeasure) otherwise than in an armed conflict would be unlawful as it would not fall within one of the exceptional circumstances envisaged by the Charter where force may be used by a State against another State. Whatever the moral argument or imperative, therefore, even the sort of contained or limited strike that the US conducted in response to the chemical weapon attack, would be unlawful if carried out as a reprisal. Indeed, given the protective nature of international humanitarian law, post-1945, even a reprisal in wartime, effected by one belligerent party to force the other to abide by the laws of war, would be tightly constrained as to target and proportionality.


1 Syria, Chemical Weapons & International Law: A Legal Snapshot

2 Colonel John Thomas, spokesman for the US Central Command

3 UK Defence Secretary, Michael Fallon

4 ‘Nothing contained in the present Charter shall authorize the United Nations to intervene in matters which are essentially within the domestic jurisdiction of any state or shall require the Members to submit such matters to settlement under the present Charter; but this principle shall not prejudice the application of enforcement measures under Chapter Vll.’

5 Military and Paramilitary Activities in and against Nicaragua (Nicaragua v. United States of America), ICJ Reports 1986

6 75 BYIL 2004

7 For a timeline of attacks allegedly involving chemical weapons, the Arms Control Association provides a useful guide which can be accessed at:




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