Editor’s Note: This is the first post in a two-part series discussing the legal justifications various countries have put forth related to airstrikes in Syria. You can find the second part here.

Last week, the UK carried out fresh airstrikes against the Islamic State of Iraq and the Levant (ISIL) in Syria. With the number of countries conducting similar airstrikes now reaching nearly a dozen, it’s worth comparing how the UK and other States have officially justified their strikes to the United Nations. (Not all countries involved in airstrikes have provided clear justifications to the UN or elsewhere, and those that haven’t are left out of the analysis below.)

In a subsequent post, I’ll set out the general rules of international law that regulate when a State can engage in such airstrikes and will reflect on the emerging legal patterns, trends, and challenges. Specifically, I’ll discuss how various countries’ airstrikes match up against the exceptional circumstances under international law that permit States to carrying out armed attacks on the territory of other States, namely 1) the consent of the host country, 2) individual and collective self-defense, or 3) UN Security Council authorization.

To set the stage, on September 20, 2014, Iraq sent a letter to the UN noting that ISIL (also known as Daesh, the Islamic State, and the Islamic State of Iraq and Syria or ISIS) had established safe havens outside Iraq’s border that posed a “direct threat to the security of our people and territory” and that those safe havens provided ISIL an ability “to train for, plan, finance and carry out terrorist operations across our borders.”

For these reasons, Iraq requested in its letter that the United States “lead international efforts to strike ISIL sites and military strongholds, with our express consent.” As a result, this letter was the doorway through which the US and others States asserted a right to collective self-defense to join Iraq’s fight against ISIL. Though, as I discuss more below, collective self-defense is not the only justification States have provided for their strikes.

United States

The first US airstrikes in Syria reportedly took place on September 22, 2014. A day later, on September 23, the United States sent its own letter informing the UN that it had “initiated necessary and proportionate military actions in Syria in order to eliminate the ongoing ISIL threat to Iraq.” The United States noted that ISIL also posed a threat to the United States and other countries. The letter further noted that the US “has initiated military actions in Syria against al-Qaida elements in Syria known as the Khorasan Group to address terrorist threats that they pose to the United States and our partners and allies.”

In the letter, the United States invoked the right to both individual and collective self-defense to justify its strikes in Syria, though in places the letter was a fuzzy as to which type of self-defense (individual vs. collective) applied to which strikes. The letter elaborated that “States must be able to defend themselves, in accordance with the inherent right of individual and collective self-defense, as reflected in Article 51 of the UN Charter, when, as is the case here, the government of the State where the threat is located is unwilling or unable to prevent the use of its territory for such attacks.”


On April 8, 2015, Canada entered into the fray by carrying out its first airstrikes against ISIL in Syria, reportedly hitting a garrison. Prior to this, on March 31, 2015, Canada wrote to the UN that ISIL “continues to pose a threat not only to Iraq, but also to Canada and Canadians, as well as to other countries in the region and beyond.” Noting that Iraq had requested a US-led coalition, Canada explained that any military actions against ISIL in Syria would be aimed at “further degrading ISIL’s ability to carry out attacks.”

Canada claimed that under “the inherent rights of individual and collective self-defence reflected in Article 51 of the United Nations Charter, States must be able to act in self-defence when the Government of the State where a threat is located is unwilling or unable to prevent attacks emanating from its territory.” Canada’s letter said its actions would be “necessary and proportionate.” Like the United States, Canada’s letter was unclear as to which type of self-defense (individual vs. collective) applied to which strikes.

All that said, Canada’s Liberal Prime Minister-designate Justin Trudeau has committed to withdrawing his country’s fighter jets from the US-led coalition against ISIL in both Iraq and Syria.


On July 25, 2015, Turkey launched its first airstrikes against ISIL in Syria, targeting two ISIL bases and a gathering point, according to the Turkish Prime Minister’s office. The day before, Turkey wrote to the UN explaining that “Syria has become a safe haven for Daesh. This area is used by Daesh for training, planning, financing and carrying out attacks across borders.” Turkey referenced a reportedly ISIL-linked July 20 attack in Suruç that killed 32 Turkish citizens as well as a border skirmish on July 23 to demonstrate that Turkey “is under a clear and imminent threat of continuing attack from Daesh.” The letter also stated that Syria is “neither capable of nor willing to prevent these threats emanating from its territory, which clearly imperil the security of Turkey and the safety of its nationals.”

Based on the right to individual and collective self-defense, Turkey said in its letter that it “has initiated necessary and proportionate military actions against Daesh in Syria, including in coordination with individual members of the Global Coalition, in order to counter the terrorist threat and to safeguard its territory and citizens.”

United Kingdom

On August 21, 2015, the UK carried out a targeted killing in Syria of an individual “known to be actively engaged in planning and directing imminent armed attacks against the United Kingdom.” The UK justified this action in a September 7 letter to the UN explaining that the strike “was a necessary and proportionate exercise of the individual right of self-defence of the United Kingdom.”

Then, on December 3 and 4, 2015, the UK launched a wave of airstrikes against oil fields in Syria with the purpose of “cutting off the terrorists’ oil revenue at the very source,” according to the Ministry of Defence. This followed a December 2, 2015 parliamentary vote in favor of strikes, which itself followed a letter to the UN from more than a year before on November 25, 2014. In that letter, the UK stated that it fully supports, and is taking measures in support of, coalition strikes against “ISIL sites and military strongholds in Syria.” The letter also stated that the UK supported such measures under the doctrine of collective self-defense of Iraq, and said the measures they were undertaking at the time were “necessary and proportionate.”

Thus, the UK first claimed a right to carry out ISIL strikes in Syria under a collective self-defense rationale, but it didn’t act on that until last week. Meanwhile, in a slightly different context, the UK carried out an ISIL strike in the name of individual self-defense months earlier.


Australia carried out its first three strikes in Syria in mid-September 2015, which reportedly targeted an oil collection point, an ISIL tactical unit, and an ISIL armoured personnel carrier. Prior to conducting the strikes, on September 9, 2015, Australia sent a letter to the UN that was similar to Canada’s. However, it only invoked the right to collective self-defense and did not go so far as to say that ISIL posed a threat to Australia. Therefore, it did not make a claim of individual self-defense. The letter states: “Australia is taking measures against the Islamic State in Iraq and the Levant (ISIL) in Syria in support of the collective self-defence of Iraq as part of international efforts led by the United States.” Australia said its military actions would be “necessary and proportionate” and it claimed Syria was “unwilling or unable” to prevent ISIL’s attacks.


On September 27, 2015, France carried out its first airstrikes against ISIL in Syria. According to Prime Minister Manuel Valls, those strikes targeted ISIL “sanctuaries where those who combat France are trained.” On November 23, 2015, following the deadly Paris attacks, France launched a fresh wave of airstrikes against ISIL in Syria (and Iraq).

On September 8, 2015, prior to the initial strikes, France sent a letter to the UN noting Iraq’s request for assistance against ISIL, emphasized the UN’s previous declarations that ISIL was a threat to international peace and security, and stated that ISIL’s actions were “a direct and extraordinary threat to the security of France.”

In rather vague terms that did not distinguish between collective or individual self-defense, France’s letter stated that “in accordance with Article 51 of the Charter of the United Nations, France has taken actions involving the participation of military aircraft in response to attacks carried out by ISIL from the territory of the Syrian Arab Republic.”


Russia launched its first airstrikes in Syria on September 30, 2015, following parliamentary approval. From the outset, Russia explained that its strikes were done with the Syria’s consent and aimed at ISIL. The New York Times reported that President Putin justified the strikes as preventive measures “to fight and destroy militants and terrorists on the territories that they already occupied, not wait for them to come to our house.” The United States and others, however, have contest that Russia is often striking non-ISIL targets in an effort to strengthen Syrian President Bashar al-Assad’s position.

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All told, Canada, Turkey, the UK, and the US invoked the doctrines of both individual and collective self-defense. Australia is the only one to have invoked only the doctrine of collective self-defense. France’s letter to the UN was ambiguous, though it highlighted general threats that ISIL poses and noted ISIL’s specific threat to France (a claim that has been made all the stronger in the wake of the Paris attacks).

Meanwhile, Australia, Canada, Turkey, and the US also referenced the controversial “unwilling or unable” test in their letters to the UN. The UK and France did not, but the UK Prime Minister did make a passing reference to the test elsewhere in relation to Syria.

Also worth noting is that the Australia, Canada, Turkey, the UK, and the US all said their actions in Syria would be “necessary and proportionate.”

Each of these claims has a specific meaning under international law, many of which I’ll explore in my next post. Suffice it to say, at least for now, that these varying claims create a complicated (and somewhat controversial) legal landscape for airstrikes in Syria.