Are US airstrikes in Syria legal?

The law of war and why it could be improved

When the United States, accompanied by Bahrain, Jordan, Qatar, Saudi Arabia and the United Arab Emirates, begun a campaign of airstrikes against the Islamic State group in September 2014, the international response was, unsurprisingly, mixed.

This use of force was of questionable legality, and its status continues to be contested to this day, owing to the obscure processes by which customary international law is formed.

New rules come about if the widespread practice by states of particular conduct (state practice) is accompanied by a claim or belief by those states that the conduct is required by law (opinio juris). So arguments about the legality of international action often depend on gathering evidence to support the claim that there is a rule allowing it. This apparently contradictory process itself is subject to a great deal of academic debate.

Taking action against the Islamic State and non-state terrorist entities more generally implicates three different controversies in international law: anticipatory self-defence; terrorist groups who perpetrate armed attacks who are not states; and the consent of the state in which the group is based.

Anticipatory self-defence

Conventional self-defence operates on a fairly straightforward basis. The general principle, embodied in article 2(4) of the United Nations (UN) Charter, is that uses of force against another state are prohibited.

Article 51 of the UN Charter currently constitutes the sole exception to this principle, allowing a state to take necessary and proportionate action against another state that has perpetrated an “armed attack” against it. While this understanding operates on the basis of one state attacking another through conventional means, the issue of decentralised and covert terrorist operations has led to demands for a doctrine of anticipatory self-defence, whereby a state could take action to respond to a sufficiently egregious attack it perceives as imminent.

There is little evidence of states acting in such a way, so the argument remains largely speculative, although a UN High-Level Panel of Experts accepted the right of anticipatory self-defence in 2004.

It is suggested the imminence requirement of this doctrine is required to limit it only to cases where taking defensive action is strictly necessary. Although the US did not expressly invoke anticipatory self-defence when launching its campaign against the Islamic State, it would make most sense for this doctrine to be adopted in relation to terrorist groups such as the Islamic State, given the latter’s numerous strikes beyond its territory in Syria and Iraq to which the doctrine could respond, and the fact that only in instances of necessity would it be justifiable to invoke self-defence and use force.

The opponent is not a state

This is perhaps the least controversial of the three issues. Although international law orthodoxy has long suggested that states are the only entities possessing a legal personality, there is ample state practice and opinio juris to suggest that non-state terrorist groups are legitimate participants in the law relating to self-defence. The US’s action against Al-Qaeda in the wake of 9/11 was the first modern example, following which the UN Security Council adopted resolutions 1368 and 1373 in endorsement of this action. Other notable examples include Russian airstrikes against Chechen rebels in Georgian territory in 2007 and the Columbian pursuit of the Fuerza Armadas Revolucionarias de Columbia into Ecuadorian in 2008. Judicially, in the Armed Activities case of 2005, Judge Simma, in his separate opinion, supported the notion of defensive action against non-state entities. There is, then, growing, albeit not universal, support for this aspect of the question.

The host state did not consent to the airstrikes

This is arguably the greatest point of contention in such situations. The right of each state to be free from any sort of interference — be it political, economic or military — is regularly posited as one of the first principles of the international legal system. This is the principle of territorial integrity, and is strongly linked to the idea of a community of sovereign states who participate in the system.

Moreover, article 2(4) of the UN Charter forbids uses of force that violate a state’s territorial integrity. The US, however, has challenged the pre-eminence of territorial integrity by coining the new doctrine of states hosting non-state terrorist groups being “unwilling or unable” to prevent those groups from carrying out attacks against other states, using the doctrine to justify both its 2011 incursion into Pakistan to eliminate Osama Bin Laden, and the commencement of its action against the Islamic State in 2014.

Ashley Deeks has argued that along with other examples in recent history (Russian action against Chechan rebels in Georgia, Turkish action against the PKK in Iraq and Israeli action against Hezbollah in Lebanon), this practice can provide the basis for finding a new rule of international law allowing a state which has suffered an armed attack to take military action against a non-state terrorist based in a host state which refuses, or lacks the capability, to quell its activities.

There are a number of problems with this idea.

As a matter of forming the rule itself, it is questionable whether this relatively limited practice can be accepted as having changed the fundamentals of the international law on the use of force. The prohibition on the use of force in article 2(4) of the UN Charter, and the sole exception in article 51, should be viewed as easily modified or overturned. Indeed, Konstantina Tzouvala has argued that the power disparity between the states invoking self-defence and those hosting terrorist entities and the excessive focus on the former in developing new legal rules reflects an overly Western and imperialist methodological preference. Certainly, the argument can be made that new customary rules ought to involve maximal formal participation in international fora — such as the UN General Assembly — to ensure, as far as possible, a global consensus in state practice and opinio juris.

It is also strongly arguable that the “unwilling or unable” standard has insufficient regard for the territorial integrity of host states, and that a more nuanced standard of complicity ought to be adopted. Despite major disparities in economic and military power between various states, the notion of the sovereign equality of states acts as a bulwark against military adventurism by more powerful states against the factually weaker. Ruys and Verhoeven — for example — have suggested adapting the international criminal law concept of aiding and abetting to allow self-defence where the host state has acted in some way so as to substantially contribute to the armed attack against the victim state. The concept has greater legal pedigree and is a better candidate for modification in the context of the use of force. It also retains the idea that the host state must have committed some offence against the victim state before self-defence can be invoked, which is preferable to the liability standard of “unwilling or unable”: an omission of uncertain contours.

Clearly, though, this standard does not solve the present example because of Syria’s continued opposition to the Islamic State, and no doubt similar fact patterns will arise in the future. But to grant too many exceptions to the general prohibition on the use of force in article 2(4), and to expose less powerful states to the risk of invasion on account of terrorist activities within their borders, would be a regressive step in international law. It would return the discipline to the 19th century paradigm whereby a small cluster of colonial powers determined the content of international law to suit their interests alone. In addition, it must be remembered that the concern in the Islamic State example is with anticipatory self-defence. The fact that a state may not even have suffered an attack before embarking on military action in another state’s territory carries self-evident dangers.

To summarise: while the traditional state-on-state approach to the law of self-defence must be developed to meet contemporary challenges, care must be taken in defining liability standards so that the values of territorial integrity, sovereignty and taking adequate action against terrorist entities are balanced. The US’s action against the Islamic State would stand a better chance of acceptance, pursuant to the formation of new rules of international law, had it based its airstrikes on the principle of anticipatory self-defence. The “unwilling or unable” doctrine, however, remains an aspect of its legal argument beyond reasonable defence.

Jonathan Mellor is a law student at the University of Oxford.