Maintaining the authority of international law has often seemed a vain hope in a world riven by conflict between the most powerful nation states. The world wide agreement after the second world war to settle disputes peacefully and through legal processes was a remarkable achievement but the Syrian catastrophe is only the latest of its many failures. Yet the need to uphold the rule of law remains as great as ever and we can be grateful that our politicians for the most part still feel the need to seek legal backing for their actions .

Is there a legal justification for the recent attack by the United States, Britain and France on Syria?

The United Nations Charter is a founding document of modern international law and its provisions are binding on all nation states. It prohibits the use of force by states except when authorised by the UN Security Council or in self- defence. Self-defence legitimises an armed response to a current or imminent attack. It does not justify the attack by the three states who have bombed Syria. Nor has the Security Council licenced them to use force. That they have violated the United Nations Charter and thus acted illegally seems incontrovertible. Yet the government has issued a legal opinion defending the legality of its actions and their arguments cannot be lightly dismissed.

In recent year the notion has been developing in international law that in extreme circumstances force may be used by states notwithstanding non-compliance with the requirements of the UN Charter. This is the evolving doctrine of “humanitarian intervention”, which has never been adopted formally in any internationally endorsed instrument but which is seen by many as a legally valid exception to the general prohibition on the use of arbitrary force.

It is on this basis that the UK government has justified the attack on Syria. It claims that three conditions necessary to justify humanitarian intervention are fulfilled. These are:

  1. Convincing evidence of extreme distress on a large scale requiring immediate and urgent relief

  2. No practicable alternative to the use of force if lives are to be saved

  3. The proposed use of force must be necessary and proportionate to the aim of relief of humanitarian suffering and must be strictly limited in time and in scope to this aim.

While there may be a broad consensus among members of the United Nations that the right of humanitarian intervention is desirable, subject to such conditions, the assertion of the UK government that it is “permitted under international law” to take the measures it has taken against Syria is problematic. The UN Security Council has itself on a number of occasions authorised humanitarian intervention but in the case of Syria it has not done so because of the exercise of the veto by Russia. Russia, of course, as a vital player in the Syrian civil war, has a powerful vested interest in opposing such intervention, conflicting with its commitment to the aims of the UN.

Should the UN and its Security Council be prevented from carrying out their proper function of securing peace between nations by one member which has a strong partisan interest? Common sense says no, even if the letter of the law suggests otherwise. If the UN cannot override the veto, should individual member states feel bound by it? The answer is surely the same.

In justifying the necessity and proportionality of the attack the UK Government relies on the repeated use by the Syrian government of chemical weapons against its own civilian population. Persuasively it claims that without forcible action of the kind taken the likelihood of the Syrian regime using chemical weapons again would be increased. This would lead to further suffering and loss of civilian life. The proportionality of the attacks is justified by their being targeted only on sites associated specifically with the development and storage of chemical weapons.

The Government also points to the continual blocking by Russia of efforts to secure impartial investigation of Syrian chemical weapons use and capability. The Organisation for the Prohibition of Chemical Weapons set up a special joint mechanism with the Security Council mandated to assign responsibility for the use of chemical weapons. When it exposed Syria as a culprit, Russia vetoed renewal of the mechanism.

Professor Marc Weller of Cambridge University has argued (BBC, 14 April) that the international prohibition on the use of chemical weapons imposed by the Chemical Weapons Convention, which Syria joined in 2013, may itself provide a justification in international law for the US, UK, and France taking it upon themselves to use force to secure Syria’s compliance. This justification would be independent of the validity and scope of any right of humanitarian intervention. Again, though powerfully arguable, the claim lacks a solid basis in any international consensus.

I am only considering here the legality of the decision to attack Syria, not the separate and highly contentious question of its political wisdom. The legal structure of the United Nations, which allows Russia a veto in the Security Council, inhibits effective enforcement of its humanitarian aims. The inevitable outcome is to drive its members to take the law into their own hands. This does not bode well for the future of international law.