The justifications put forward by the US, UK and France for the air strikes in Syria have focused on the need to maintain the international prohibition against the use of chemical weapons, to degrade President Assad's chemical weapons arsenal and to deter further chemical attacks against civilians in Syria.
Prime Minister Theresa May argued that the UK has always stood up for the defence of global rules and standards in the national interest of the UK and of the organised international community as a whole.
However, in its formal legal defence of the operation issued some time later, the UK government rather emphasises the need to protect the population of Syria from further harm.
Legally, the claim to enforce international law on chemical weapons by violent means would return the world to the era before the advent of the UN Charter. The Charter allows states to use force in self-defence and, arguably, for the protection of populations threatened by extermination at the hands of their own government. The use of force for broader purposes of maintaining international security is also possible. However, such action is subject to the requirement of a mandate from the UN Security Council.
This arrangement tries to balance the need of states to preserve their security in the face of an actual or imminent attack through self-defence when strictly necessary with the need to ensure that force cannot be used as a routine tool of international politics. Hence, international law since 1945 precludes military strikes in retaliation - to teach other states a lesson, as it were - or by way of reprisal. Reprisals are acts that are in principle unlawful, but they can be excused because they aim to force a state back into compliance with its international obligations.
Hence, in 1981 Israel was condemned by the UN Security Council when it attacked the Osirak nuclear reactor in Iraq. Israel had argued that it might contribute to the production of weapons of mass destruction in the future. A US attack against an alleged chemical weapons facility in Sudan in 1998 in response to US embassy bombings in Kenya and Tanzania was also criticised.
In this instance, the three states mounting the air strikes have taken it upon themselves to force Syria into compliance with its obligations under the Chemical Weapons Convention. Syria joined the Convention in 2013 as part of the diplomatic settlement that followed the failure of the UK, and the US, to go through with threatened air attacks after gruesome chemical attacks in Eastern Ghouta. The Convention prohibits the production, possession and use of chemical weapons. No fewer than 192 states have signed.
Syria was also subjected to additional duties contained in mandatory Security Council resolution 2118, reinforcing these obligations and providing for the destruction of its chemical weapons stockpile. In an impressive example of international co-operation, also involving Russia, this was largely achieved a year later, by September 2014.
However, since then, there have been some 40 recorded instances of alleged chemical weapons use in Syria. The Organization for the Prohibition of Chemical Weapons (OPCW) has the ability to dispatch fact-finding missions to determine whether such weapons have actually been used.
A special joint mechanism was set up by the OPCW and the Security Council with a mandate to assign responsibility for such uses. However, after the mechanism pointed the finger at the Assad government last year, Russia vetoed its renewal.
An attempt to establish a new mechanism empowered to determine responsibility for the latest use of chemical weapons in Douma failed this week, again due to a Russian veto in the Security Council. Russia's own proposed investigatory mechanism, which was opposed by the Western states and others, would have lacked that power.
The three states intervening in Syria now argue that there was no prospect of obtaining a mandate from the Council to confront chemical weapons use by Syria. In striking Syria, they claim to have fulfilled an international public order function of defending the credibility of the prohibition of the use of chemical weapons in general terms, and enforcing Syria's obligations in particular.
This argument is somewhat reminiscent of the 2003 invasion of Iraq, supposedly to enforce Baghdad's disarmament obligations imposed by the Security Council in the absence of clear Security Council authorisation. Moreover, in April of last year, President Trump launched 59 cruise missiles against the Syrian air-base at Shayrat. It was claimed that the installation had been involved in a chemical attack in the town of Khan Sheikhun, again to restrain further chemical weapons use.
The blockage in the Security Council on Syria opens up some space for this kind of argument. The Chemical Weapons Convention provides for referral of grave instances such as the Douma attack to the Security Council for enforcement action. But the Council could not even agree on a mechanism to establish responsibility, not to speak of more decisive action to repress future uses of such weapons.
The claim of the three states involved to act instead of the Council, as the world's enforcement agent of a highly important international rule, is of course being resisted by some. Russia has already asserted that the attacks flagrantly violate the prohibition of the use of force. The UN secretary general has also emphasised the need to respect the primacy of the Security Council.
The arrogation of the functions of the Council by a group of states claiming to act in the common interest therefore reflects the reality of the present, little Cold War between Russia and the West. The breakdown of the consensus that facilitates the operation of collective security necessarily results in unilateral acts and, in consequence, further division.
In addition to the general interest in maintaining the obligation to refrain from chemical weapons use, Mrs May also referred to the protection of civilians from further chemical attacks to alleviate further humanitarian suffering. This, in fact, is a stronger and more persuasive legal argument in favour of the strikes.
In fact, the formal legal argument put forward by the UK government sometime after the prime minister addressed the nation does not rely on the claim to enforce the terms of the chemical weapons convention or of resolution 2118.
In 2013, when the use of force was expected after the Ghouta attack, the UK already expressly invoked the doctrine of humanitarian intervention. A good argument has been made that states can act in cases of overwhelming humanitarian necessity that cannot be addressed by any other means to protect populations in danger of imminent destruction.
The doctrine of forcible humanitarian action gained credence throughout the 1990s when it was applied to rescue the Kurds of northern Iraq and the Marsh Arabs in the south of Iraq from destruction by Saddam Hussein. It was later employed unopposed in cases including Liberia and Sierra Leone.
However, international division about its application emerged in the wake of the operation on behalf of the Kosovo Albanians of 1999.
Since then, the UN has embraced the concept that international action can be taken to rescue a population under immediate threat. However, the doctrine of responsibility to protect (R2P) was narrowed down to cover operations mandated by the Security Council. Still, a number of states claim a right to act when the Council cannot.
Innocent civilians at risk of destruction should not pay the price for a political blockage at UN headquarters in New York. Others, including Russia and China, oppose humanitarian intervention.
The application of this doctrine is not restricted to uses of chemical weapons against civilian populations. However, given the uncontrollable and indiscriminate effect of chemical weapons, their use against civilians offers perhaps the clearest trigger for the application of this doctrine.
In this instance, the UK points to the history of repeated uses of chemical weapons by the Syrian government, adding that it is "highly likely" that further such attacks would follow. Given the failure of previous attempts to rein in this practice, and the present blockage in the Security Council, it is argued that no means other than the use of force were available to secure the humanitarian objective of the operation.
Moreover, it is asserted the force used was strictly limited and specifically targeted at objects connected with the objective of degrading the capacity to launch future chemical attacks and deterring such attacks in the future. These arguments conform to the legal requirements for humanitarian intervention.
It could also be argued that the attacks aim to preserve the national security of the states involved in the attacks, by way of an extensive right to self-defence. Diluting the protection stemming from the prohibition of the use of chemical weapons in recent practice posed a threat to all states.
Of course, every state may defend itself, under some circumstances even before an armed attack aimed at it has landed on its territory. But the attack must be imminent, leaving no choice of means and the response must be proportionate to the attack.
In the run-up to the Iraq war of 2003, there was the famous 45-minute claim concerning Iraq's supposed weapons of mass destruction. Laying the ground for an argument of anticipatory self-defence against a strike that might come in the future, the UK argued that Iraqi weapons of mass destruction might reach UK military bases in Cyprus with minimum warning.
But there was no evidence that Baghdad was contemplating such an attack and the argument was abandoned. Similarly, there is no suggestion in this instance that Syria was preparing to launch an attack against the US, UK or France.
Marc Weller is Professor of International Law in the University of Cambridge and the editor of the Oxford University Press Handbook on the Use of Force in International Law.