The U.S. Airstrike After the Use of Chemical Weapons in Syria: National Interest, Humanitarian Intervention, or Enforcement Against War Crimes?

Mika Hayashi
July 13, 2017

The chemical attack in Khan Shaykhun, Syria, reported on April 4, 2017, produced 86 deaths and more than 300 injuries according to the initial reports.[1] The Fact-Finding Mission of the Organisation for the Prohibition of Chemical Weapons (OPCW) was able to deploy in less than twenty-four hours, and later confirmed the incident,[2] even though Syria “categorically reject[ed] the false accusations and allegations of the Syrian Arab Army’s use of toxic chemical substances against Syrian civilians in Khan Shaykhun.”[3] In response to this chemical attack, a total of fifty-nine cruise missiles were launched from two U.S. naval vessels, targeting the Al-Shayrat military airbase which, according to the United States, had been used for the chemical attack by the Syrian government that had taken place three days earlier.[4] The UN Security Council that met immediately after the chemical attack had made no decision to authorize the use of force.[5]

The legal assessment of such a response is not immediately clear. On the one hand, there is a growing consensus on the illegality of the use of chemical weapons.[6] On the other hand, that does not automatically make a unilateral military intervention in response a lawful action. Given the prohibition of the use of force under the UN Charter and the narrow and limited exceptions it sets out, it is crucial that clear justifications be offered by the state carrying out a military intervention, and that such justifications be accepted by the international community. In respect to the Syria events, the United States and the states that chose not to criticize the U.S. airstrikes rooted their positions in different legal theories. This Insight examines how states responded, focusing on three types of justification: self-defense, humanitarian intervention, and enforcement against war crimes.

Could the U.S. airstrike on April 7, 2017, be justified as the exercise of right to self-defence, as the action was said to be in the “vital national security interest” of the United States?[7] According to the U.S. president and his spokesperson, the idea behind this justification was that leaving the situation in Syria unattended produced a prospect or possibility of proliferation of chemical weapons and/or that these weapons may be directed at the United States at some point in the future: “The reason that we took action was multifold—number one, to stop the proliferation and deterrence of chemical weapons. When you see mass weapons of destruction being used it should be a concern to every nation, especially our own people. The proliferation of those weapons pose [sic] a grave threat to our national security. So, number one, we have to stop that.”[8] This resonates with what the United States had advanced as a reason for a military intervention in 2013.[9]

However, self-defense is conditional upon the existence of an imminent threat. The perceived threat of proliferation, or an eventual attack on the United States, can hardly be construed as an imminent threat as required by the conventional doctrine of self-defense or by Article 51 of the UN Charter. Presenting a remote future scenario as a threat in the context of right to self-defense proved to be very controversial, as the debate about the preemptive doctrine in the past vividly illustrates.[10]

What is striking in the present case, though, is that the majority of states that chose not to criticize the U.S. airstrike appear to be uninterested in this line of argument. Indeed, very few states even refer to the U.S. “vital national security interest” in the form of the prevention of chemical weapons’ proliferation as the reason for accepting the U.S. airstrike.[11] On the whole, the threat of proliferation as a justification is not selling.

The second line of argument is humanitarian intervention. The U.S. airstrike has also been described by the White House spokesperson as “a clear response on humanitarian purposes,”[12] and the message by the U.S. president cited previously[13] indicates that a humanitarian concern had a significant role in the decision to carry out the airstrike. This notion that the scale of the humanitarian distress caused by the chemical attack justified, at least partially, the U.S. airstrike found much more purchase in the reactions of other states. For example, instead of condemning the airstrike, the German Chancellor stated that the U.S. airstrike was “understandable” given, among other factors, the suffering of innocent people.[14] The Italian foreign minister was of a similar view.[15] The United Kingdom stated in a Security Council meeting that the airstrike was a response “to unspeakable acts that gave rise to overwhelming humanitarian distress.”[16]

While none of these statements and remarks, including those of the United States, explicitly refer to humanitarian intervention as a legal doctrine and justification, the doctrine is the only legal language currently available that can accommodate the expressed sentiments. Moreover, humanitarian intervention was raised as a possible justification by the United Kingdom in 2013[17] when it was considering a possible military intervention because of the chemical attack in Ghouta, Syria.

However, the doctrine of humanitarian intervention cannot be upheld as a rule of positive law lightly. It remains very controversial as a doctrine to give lawful cover to a unilateral military intervention without the Security Council’s authorization in the case of overwhelming humanitarian distress.[18]

In the Syria situation, there appears to be a third, emerging idea that made the U.S. airstrike acceptable to other states: the enforcement of international law in the face of a violated rule. A short survey of remarks by states and other international actors indicates that the U.S. airstrike was accepted or endorsed as a form of enforcement of the rule prohibiting the use of chemical weapons. In support of the U.S. airstrike, the president of the EU Commission stated that “the repeated use of such weapons must be answered” and that the U.S. airstrikes “seek to deter further chemical weapons atrocities.”[19] The statement of the NATO Secretary-General immediately after the airstrike also stressed that “any use of chemical weapons is unacceptable, cannot go unanswered, and those responsible must be held accountable.”[20] The joint communiqué issued by France and Germany on the same day also supported the U.S. airstrike, emphasizing that the “continued recourse to chemical weapons and mass crimes cannot remain unpunished.”[21] The British Permanent Representative to the UN stated that “the United Kingdom supports the U.S. air strike on the Al Shayrat airfield because war crimes have consequences.”[22]

Such a justification for the U.S. airstrike does not, of course, signal a general acceptance of unilateral enforcement against any violation of any rule. The first important qualification, recurrent and consistent in various statements in the present case, is that this airstrike was a measure against a war crime, or a crime of such gravity that it cannot be left unattended. The second qualification, again fairly consistent across various statements, is that this particular unilateral measure was, and must remain, an interim measure pending a resolution in a multilateral framework, in particular at the United Nations.

The idea that an airstrike can be lawful when construed as an enforcement measure against a war crime also raises fresh questions. Viewed as a sign of an emerging international consensus, the present case is intriguing because the United States did not put it forward. It is those who reacted to the U.S. airstrike that expressed this idea. In the doctrine of customary international law, state practice and opinio juris as the evidence of customary rule are supposed to be concomitant. From that perspective, it may be peculiar to view the present case as part of a rule formation. States commenting on the U.S. actions appear to be providing an opinio juris, while the state that acted, the United States, is providing evidence of state practice without explicitly sharing the same opinio juris. Nevertheless, it would seem that if the state that acts does not express any subsequent view that contradicts opinio juris expressed by the commenting states, the agglomeration of opinio juris and state practice in this situation may bolster the case for a rule formation. Needless to say, the remaining conditions in the traditional customary international law doctrine must be met in order to argue that there is in fact a customary rule.

There is also a technical but important question regarding the use of the term “war crime.” Was it merely a rhetorical device, or did states truly mean that the use of chemical weapons in a non-international conflict, such as their use by the Syrian armed forces in the present case, is a war crime under international law? In this regard, the amendment to the ICC Statute adopted by consensus in 2010 included the addition of the use of chemical weapons to the list of war crimes under Article 8(2)e.[23] This may amount to evidence that at least eighty-four states were inclined to view the prohibited use of chemical weapons in a non-international armed conflict as a war crime.

In conclusion, none of the three theories explored as justifications for a military intervention garnered an international consensus in the Syria situation. In the unmistakable presence of divergent views about the case and the rules among various actors, it is certainly not possible to confirm the existence or emergence of a rule allowing a third state to resort to use of force as a response to the prohibited use of chemical weapons in a non-international armed conflict. Given this context, it is all the more important to know what these divergent views were, and to discern the ideas that may lead to an emergence of a new consensus.

About the Author: Mika Hayashi is a professor in the Graduate School of International Cooperation Studies (GSICS) in Kobe University.

[1] The information appears in the Briefing to the UN Security Council on the situation in Syria by the Under-Secretary-General for Political Affairs, UN Dept. for Political Affairs (Apr. 7, 2017),

[2] Ahmet Üzümcü (OPCW Director-General), Note by the Technical Secretariat: Status Update of the OPCW Fact-finding Mission in Syria Regarding a Reported Incident in Khan Shaykhun, 4 April 2017, U.N. Doc. S/2017/440 (May 19, 2017).

[3] U.N. SCOR, 72d Sess., 7915 mtg, U.N. Doc. S/PV.7915 (Apr. 5, 2017) [hereinafter Apr. 5 Security Council Meeting].

[4] U.N. SCOR, 72d Sess., 7919 mtg, U.N. Doc. S/PV.7919 (Apr. 7, 2017) [hereinafter Apr. 7 Security Council Meeting].

[5] Apr. 5 Security Council Meeting, supra note 3.

[6] Chemical Weapons Convention art. 1, Sept. 3, 1992, 1972 U.N.T.S. 45. For the customary rule prohibiting the use of chemical weapons, see Masahiko Asada, A Path to a Comprehensive Prohibition of the Use of Chemical Weapons under International Law: From The Hague to Damascus, 21 J. Conflict & Sec. L. 153 (2016). See also various statements in the Apr. 5 Security Council Meeting, supra note 3.

[8] Daily Press Briefing by Press Secretary Sean Spicer, The White House (Apr. 10, 2017),

[9] Mika Hayashi, Reacting to the Use of Chemical Weapons: Options for Third States, 1 J. on Use of Force & Int’l L. 110 n.154 (2014) (citing President Obama: “I Have Not Made a Decision” on Syria, PBS (Aug. 28, 2013),

[10] Id. at 110–12.

[11] Japan appears to be the only state among the ones examined below that explicitly sympathized with the U.S. concern for the proliferation of chemical weapons. Comment by Prime Minister Shinzo Abe on the Situation in Syria at a Press Occasion, Prime Minister of Japan and His Cabinet (Apr. 7, 2017),

[12] Press Gaggle by Press Secretary Sean Spicer, The White House (Apr. 7, 2017),

[13] Sampathkumar, supra note 7.

[14] Assad trägt alleinige Verantwortung [Assad Bears Sole Responsibility], Die Bundeskanzlerin [The Chancellor] (Apr. 7, 2017), The German Chancellor also refers to the state of the Security Council on this issue and the continued use of chemical weapons as two other elements that make the U.S. action “understandable.”

[15] A statement given to the press is also available at the website of the Ministry, Siria [Syria], Ministro degli Affari Esteri e della Cooperazione Internazionale [Ministry of Foreign Affairs and International Cooperation] (Apr. 7, 2017), The statement of Italy in the Security Council meeting later on the same day confirmed this statement. Apr. 7 Security Council Meeting, supra note 4.

[16] Apr. 7 Security Council Meeting, supra note 4.

[17] See Hayashi, supra note 9, at 115 n.193.

[18] Christine Gray, The Use of Force for Humanitarian Purposes, in Research Handbook on International Conflict and Security Law: Jus ad Bellum, Jus in Bello and Jus post Bellum 231-40 (Nigel D. White & Christian Henderson eds., 2013).

[19] European Commission Press Release, Statement by the President of the EU Commission on the Situation in Syria (Apr. 7, 2017),

[20] Statement by NATO Secretary General Jens Stoltenberg on US Strikes in Syria, NATO (Apr. 7, 2017),

[21] Communiqué franco-allemand sur les frappes américaines en Syrie [French-German Statement on US Strikes in Syria], La France en Allemagne [France in Germany] (Apr. 7, 2017), and Gemeinsame Erklärung von Bundeskanzlerin Merkel und Frankreichs Präsident Hollande nach den Luftschlägen in Syrien [Joint statement by Chancellor Merkel and France's President Hollande After the Air Strikes in Syria], Die Bundeskanzlerin [The Chancellor] (Apr. 7, 2017),

[22] Apr. 7 Security Council Meeting, supra note 4.

[23] Amendment to Article 8 of the Rome Statute of the International Criminal Court, June 10, 2010, 2868 U.N.T.S. 195.