On 26 June 2017, the Government of the United States of America issued a statement to inform that Syria appeared to be preparing a chemical attack, and to threaten the use of force if one took place. Earlier, on 28 May, the President of France similarly declared to be ready to use force as a reprisal against the use of chemical weapons by Syria. Both declarations followed the actual military intervention of 7 April, when the United States launched a missile strike against the Syrian airbase of Shayrat, claiming to be acting in response to an alleged chemical attack by Syrian forces in Khan Shaykhun.
Most commentators defined the attack as a clear violation of international law, perpetrated in the absence of self-defence justification and without any Security Council authorisation, which – as detailed below – are the only two exceptions to the prohibition to use force foreseen by the UN Charter. Conversely, the vast majority of States condemned the use of chemical weapons allegedly used by Assad, but not the forcible countermeasure taken against it.
Several observers and scholars argued that the silence of the international community on the violation of the prohibition to use of force is shaping a new customary norm, allowing States to use force in case of grave violations of international law. This post will question this assumption, which bears the risk of a dangerous rift in the jus ad bellum regime.
The Attacks in Khan Shaykhun and Shayrat and the Reactions of the International Community
With the bombardment of Shayrat on 7 April 2017, the US directly used force, for the first time, against the Syrian army. The US presented the attack as a reaction to the alleged use of Sarin gas by Syrian forces in an airstrike in Khan Shaykhun three days earlier. The parties to the conflict did not agree on the dynamics of the events and no independent investigation confirmed the use of chemical weapons by the Syrian government, despite the mandate of the Organization for the Prohibition of Chemical Weapons-United Nations Joint Investigative Mechanism to monitor their use in Syria. The Organisation for the Prohibition of Chemical Weapons did find traces of Sarin in Khan Shaykhun, but could not state which party to the conflict was responsible for its use. The US and French intelligence blamed the Syrian Government for the attack, whereas Russia, and Syria itself, denied any responsibility of Assad’s Government.
The US attack was in fact criticised as an act of aggression by Syria, by its main allies in the area, Russia and Iran, and other States like North Korea. The legitimacy of the attack was firstly affirmed by the US and its allies in the area. Interestingly, most States not involved in the conflict criticised Syria for using chemical weapons, but not the US aggression in itself. US and French statements issued in the following months reiterated this position. This may suggest an acquiescence towards the legality of limited military interventions, or rather just a certain diplomatic tolerance for limited interventions, which are qualified as legitimate but not legal.
The prohibition to use force in international law and its exceptions
The prohibition to use force against the sovereignty of States is the cornerstone of the UN Charter, which prohibits the use of force at art. 2(4): “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 any other manner inconsistent with the Purposes of the United Nations”. The Charter foresees two limited exceptions: when the UN Security Council authorises States to take “action by air, sea, or land forces as may be necessary to maintain or restore international peace and security” (art. 42), and on the ground of self-defence (art. 51). The prohibition was reaffirmed by the International Court of Justice in several cases, and it may amount to a jus cogens norm.
The analysis of the compliance of the attack with the UN Charter is therefore uncontroversial: the US did not claim self-defence or seek a Security Council authorisation. The attack was consequently carried out in violation of the Charter.
However, a school of thought advocates the legitimacy of a third exception to the prohibition to use force. In the absence of self-defence claims and Security Council authorisation, a military intervention would be legitimate to respond to gross violations of international law involving atrocities perpetrated against civilian populations. The debate on the existence of such an exception usually refers to doctrine of humanitarian intervention. This doctrine has no generally shared definition – let alone recognition – in international law, but it is considered a particular form of use of force in a foreign State, characterised by a) the purpose to stop or oppose mass atrocities; b) the lack of consent of the State c) the absence of a legal mandate from the UN Security Council. Humanitarian intervention does not have a clear legal basis or a defined scope, but it is considered to involve a major military commitment, which comprehensively addresses the humanitarian crisis. In addition, humanitarian intervention concerns mass atrocities against civilian populations, which have taken place in Syria during the conflict, but without provoking interventions. Thus, the case at hand would not fall within the debate on humanitarian intervention – which was not invoked by the U.S. to justify the attack. The attack of 7 April on the Syrian base of Shayrat would rather constitute a single episode of forcible countermeasure against the use of chemical weapons.
Forcible countermeasures in international law
In cases of international wrongful acts, international law allows affected States to adopt countermeasures, subject to various limitations (e.g., the existence of a breach, the need of a prior demand for reparation, the necessity to comply with proportionality), including the prohibition to use force, as clearly stated in art. 50 the Draft Articles on Responsibility of States for Internationally Wrongful Acts, adopted by the International Law Commission (ILC) in 2001. Thus, forcible countermeasures, sometimes referred to as reprisals, are in principle illegal under international law.
In the aftermaths of the 1998 NATO military intervention in Serbia, Antonio Cassese – inter alia the first President of the ICTY – argued that a new international customary norm was in statu nascendi, modifying the status of the prohibition of forcible countermeasures in international law. According to this author, in cases of gross violations of human rights, the use of force as a countermeasure could be allowed, under certain conditions. Cassese acknowledged that this sort of derogation from the regulation of the use of force did not yet exist in international law because of the lack of sufficient State practice, while there was already an opinio iuris ac necessitatis, given the diplomatic position of the majority of States concerning the NATO intervention.
Almost twenty years later, in the light of the development of the approach of States to the use of force, a consistent State practice did not evolve. The silence of States could arguably express an acquiescence to the adoption of forcible countermeasures against gross human rights violations. Even in this case, it would constitute an exception, and State practice does not show any crystallization of a similar customary norm.
In case of use of force without any Security Council authorisation, States have mostly relied on an extensive interpretation of the principle of self-defence. In contrast, States do not usually claim the possibility to use force as a reprisal for a violation of human rights.
Furthermore, in the last decades, the international community’s approach against military interventions like the bombing of Shayrat was strengthened by the criminalisation of the act of aggression. The crime of aggression was included in the Rome Statute of the International Criminal Court with a series of amendments at the 2010 Review Conference in Kampala. The ICC Assembly of States Parties is currently preparing to activate the Court’s jurisdiction over the crime. In its definition, the crime of aggression clearly covers actions like the 4 April attack on Syria.
To conclude, the statements against the use of chemical weapons by Syria raised the debate on the emergence of an opinion iuris allowing forcible countermeasures in case of gross violations of human rights. Still, the scarcity of State practice hinders the creation of a new customary norm which is capable to infringe the UN system of jus ad bellum. States do not seem ready to permit derogations from the prohibition to use force. In the lex lata, the bombardment of Shayrat, and the following threats to further use force against Syria constitute a clear violation of the jus ad bellum.
The absence of an explicit condemnation, without any consistent State practice, does not risk creating a customary derogation to the prohibition to use force as enshrined in the UN Charter with a new customary norm. Rather, it may suggest a certain diplomatic tolerance for limited interventions, which are sometimes defined as legitimate but not legal. This does not question the prohibition to use force, which is the key international rule to protect international peace and security.
Antonio Cassese, ‘Ex iniuria ius oritur: Are We Moving Towards Legitimation of Forcible Humanitarian Countermeasures in the World Community?’  10(1) European Journal of International Law 23-30
 Antonio Cassese, ‘A Follow Up: Forcible Humanitarian Countermeasures and Opinio Necessitatis’  10 (4) European Journal of International Law 791-799; Bruno Simma, ‘NATO, the UN and the use of force: legal aspects’  10(1) European Journal of International Law 1-22.