US Cruise Missile Strike in Syria and Implications for International Law

By Toby Fenton

A US guided-missile destroyer launches a Tomahawk cruise missile during NATO’s intervention in Libya, 2011. Image Credit: Wikicommons | U.S. Navy photo by Mass Communication Specialist 3rd Class Jonathan Sunderman/Released


The US approach to Syria has entered a new phase. On 4 April, a suspected chemical attack [i] occurred in Khan Sheikhoun, a town in Syria’s Idlib province; more than 80 people have reportedly been killed, with many others suffering from related symptoms. According to the US, UK and NATO, Syrian fighter jets conducted a chemical weapons airstrike (scenario 1). According to Syria and Russia, Syrian fighter jets conducted a conventional weapons airstrike against a rebel-operated chemical weapons storage site, with the impact resulting in the dispersal of toxins (scenario 2). On 7 April 2017, two US Navy vessels in the Mediterranean launched 59 Tomahawk cruise missiles at a Syrian airbase alleged to be the launch site for the suspected chemical attack. The missile strike reportedly killed at least 6 people and destroyed the facility. The use of chemical weapons is a serious violation of international law, and for many observers, and the US response was justified. Yet the US strike itself has serious implications.

The suspected chemical attack

While news reports convey the aftermath of the suspected chemical attack, it appears too early to conclusively say what happened. Open-source researchers have corroborated the likelihood of the first scenario; and a number of chemical weapons experts say that if the targeted building had housed chemical weapons — as the Syrians and Russians claim — these would have been obliterated, not released, by the airstrike. However, this first scenario is not without weaknesses. Pending further investigation, it remains possible that the targeted building was indeed a chemical weapons storage site, and that some toxins on-site might not have been entirely obliterated during an airstrike but could have instead been released and dispersed through the impact.

While the purpose here is not to analyse various hypotheses of the incident, there is currently an absence of strong material evidence that would allow for the conclusive dismissal of either of the above scenarios. This means there is a danger that the US decision to strike at this stage that may set a problematic precedent, i.e. the resort to force as a punitive measure requires a relatively low threshold of evidence. On the other hand, even if Syrian forces conducted a chemical attack, the US response itself is legally problematic.

The US cruise missile strike

The use of force by one state against another state is prohibited under Article 2(4) of the UN Charter. The two Charter exceptions to this is force that is a) authorised by the UN Security Council, and b) used in self-defence. Neither of these conditions was satisfied by the US cruise missile strike.

The Security Council did not authorise the use of force in Syria. The Security Council resolution drafted by the US, UK, and France following the suspected chemical attack did not call for military action and has not (yet) been put to a vote. Countries engaging in military intervention without explicit Security Council permission at the time have occasionally sought to justify their intervention by reference to one or more prior Security Council resolutions that explicitly or implicitly mandated future military force — for instance, if certain conditions remained unfulfilled or certain obligations were subsequently violated. However, previous resolutions on the Syrian conflict have not contained such a provision for the use of force [ii].

The conditions for the exercise of self-defence were also not met. In particular, one of the jus ad bellum conditions for self-defence is that the use of force be ‘necessary’, as determined by the ‘imminence’ of an attack. The US (including any US troops in Syria) was not attacked by Syria, nor is there any allegation that such an attack was either planned or imminent.

The US has not justified the cruise missile strike in legal terms but in terms of responding to an unacceptable act (the suspected chemical attack). The US (and the UK) have, however, emphasised that the strike was “limited” and “proportionate”, perhaps implying that it met the jus in bello principle of ‘proportionality’. However, if the use of force does not have Security Council authorisation and does not qualify as self-defence, the claimed proportionality of that force is insufficient to impute lawfulness. In other words, while the particular application of force — cruise missiles against the Syrian airbase — may have been proportional to the alleged need to destroy Syria’s means of conducting potential chemical weapons attacks, the initial resort to force per se likely remains unlawful.

The US missile strike does not appear to meet the criteria for a case of ‘illegal but legitimate’ military action à la Kosovo. The strike was conducted with “no apparent international support”, and President Trump’s statement following the strike framed it primarily in terms of advancing or protecting US national interests, rather than being spurred by humanitarian purposes.

Resolve or escalation?

The use of chemical weapons is a violation of international law, prohibited under several international agreements. By virtue of their relatively indiscriminate nature and dispersed effects, their use — particularly in a civilian-populated area — would likely constitute a de facto war crime and violation of the international humanitarian law. However, the Security Council retains exclusive competency to authorise military force in response; any unauthorised response by force is likely to be unlawful, regardless of the original illicitness of the use of chemical weapons.

Syria’s critical ally Russia has condemned the US missile strike as an act of aggression and a violation of international law. Moscow has suspended the Russia-US flight safety agreement that was intended to prevent mid-air collisions between Russian and US military aircraft over Syria. Moreover, it has pledged to reinforce Syria’s anti-aircraft defences. Even if the intent was to demonstrate resolve against a violation of international law in one sense, the US cruise missile strike on Syria may have violated and weakened it in another sense — while escalating an incredibly volatile situation.

Toby Fenton is currently pursuing a Master’s degree in International Peace & Security at King’s College London.


[i] This terminology follows the BBC’s practice (at the time of writing), pending clarification of the incident.

[ii] Security Council Resolution 2249 (2015) called on Member States to “take all necessary measures” against ISIL/Da’esh (aka Islamic State), but did not actually authorise the use of force under Chapter VII on the UN Charter.

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1 thought on “US Cruise Missile Strike in Syria and Implications for International Law”

  1. Sadly, there parts of international law have been non-functional since the invasion of Iraq. We’re back to the pre-WWI situation as regards states attacking each other.

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