Most international lawyers have said that the US missile strikes against the Shayrat airfield in Syria on Friday morning were unlawful. The UN Charter prohibits recourse to force except in self-defence or if authorised by the Security Council to maintain international peace and security. The airstrikes, undertaken unilaterally in response to a chemical weapons attack allegedly conducted by the Syrian government against Syrian civilians, do not appear to fall within the limited exceptions of collective security or self-defence. The US government has given no legal justification for its actions. Yet many US politicians, Western allies and liberal commentators have supported the airstrikes, seemingly untroubled by the implications of the Trump administration’s nonchalant disregard for international law.

The prohibitions on the use of force and on intervention in the domestic affairs of other states are seen as foundational to the modern international legal order. Governments have felt obliged to account publicly for the lawfulness of their actions, to distinguish permissible state force from aggression, genocide, war crimes or arbitrary killing. The tendency to engage in ‘lawfare’ has been especially marked on the part of lawyers for such militarily interventionist states as Russia and the US. International law is one of the few modern legal systems that retains the notion of custom or practice as a source of law. Interventionist states and their lawyers often assert that their interventions are justified by a change in the law, rather than representing a breach of it. The US administration’s appeals to the doctrine of pre-emptive self-defence are one example; the expansive interpretations of the legality of intervention by invitation used by all states intervening in Iraq and by Russia and Iran in Syria are another.

The absence of any legal justification in the statements about the airstrikes made by President Trump and his officials is significant. The Trump administration has already shown disdain for domestic judges and courts, and seems not only unconcerned by the possibility that its policies might violate international agreements, but committed to the tearing up of existing treaties. Before Friday’s airstrikes, many liberal internationalists expressed concern about the implications of this for the world order that has been under construction since the 1940s.

Yet many Democrats, including former Obama administration officials, appear supportive of the Trump administration’s penchant for unilateral military action. Harold Koh, a former legal adviser to the State Department, said that the airstrikes were ‘not illegal’, and that an ‘important moment of lawmaking’ is ‘now upon us’, requiring ‘not just bombs, but diplomacy’. Anne-Marie Slaughter, the director of policy planning at the State Department under Hillary Clinton, said: ‘Donald Trump has done the right thing on Syria. Finally!! After years of useless handwringing in the face of hideous atrocities.’ Such responses from establishment liberal lawyers should give us pause.

The international legal order is far from ideal. But it provides at least some means by which the justifications for the use of state violence can be opened to public debate and challenge. The institutional culture of collective security requires states to provide an ongoing interpretation of the actions they take in defence of individual and collective security, while recognising that their interpretations may set precedents for the future.

The US government seems set on moving beyond even this minimally constraining legal order. It offered no doctrine or even coherent policy to explain its resort to force against Syria, other than Trump’s disquieting appeals to the battle between ‘civilised nations’ and barbarism, to ending the suffering of the ‘children of God’, and to the faith that ‘as long as America stands for justice, then peace and harmony will, in the end, prevail.’ A command-and-control model of international law and order was grafted onto a story of good and evil and a hugely mediatised mobilisation of sentimentality.

There is strong evidence that Assad’s regime has engaged in devastating attacks on civilians, but it also seems clear that reporting from Syria is selective and shaped by the Western media’s dependence on rebel groups. The Trump administration has taken on the role of determining when the facts in foreign countries warrant punitive reprisals or military action in the name of ending bloodshed. This has been welcomed by liberal internationalists in the US and allied states, who appear to see in Friday’s airstrike the potential realisation of their dreams for a more muscular humanitarianism. The challenge for international lawyers committed to a pluralist world order is to show how, under such conditions, the articulation of competing values and interests in the public language of law can be preserved.