UK participation in Syrian missile strike on shaky legal ground

The PM continued that there is "no practicable alternative to the use of force to degrade and deter the use of chemical weapons."

Her justification is that the strike is "specifically about deterring the Syrian regime" and sending "a clear signal" to others who might use chemical weapons.

May claimed that the action is "in Britain's national interest".

Unfortunately, these claims do not withstand scrutiny.

First, the British public does not see the military strikes as being in the national interest: 43 percent opposed missile strikes whereas only one in five people supported the use of cruise missiles against Syria.

The prime minister has not explained which national interests are impacted by Assad's use of chemical weapons.

No British citizens were harmed by their use last week and no vital security interests were affected.

Although the use of chemical weapons is reprehensible, their use in Syria does not impact any specific British national interest.

Secondly, May's argument for the use of force to specifically deter Assad has no support in international law.

The UN Charter does not authorise states to use force for deterrence. Specifically, article 2(4) states: "All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the purposes of the United Nations."

The authority for using force is limited to the UN Security Council. In addition to such collective force, states may only use force in self-defence.

The charter vests the UN Security Council (SC) with the right to use force under article 42.

The SC may use force if other measures to give effect to its decisions "would be inadequate or have proved to be inadequate".

The use of force includes actions "by air, sea, or land forces as may be necessary to maintain or restore international peace and security."

Russian veto at UN

In this case, the UN Security Council has not authorised the use of force under article 42. There is no prospect of such authorisation due to Russia's veto power.

The right to self-defence is recognised in article 51: states can defend themselves if there is an "armed attack" against them. There is no "armed attack" against Britain.

May also claims that "there is no practicable alternative' to the use of force. However, this is simply not true.

The chemical weapons attack has already transpired. There is no intelligence that a new attack is imminent – necessary for a preventative justification.

So, this missile strike is not impelled by a need to save human lives where there is no other alternative.

There are other alternatives to missile strikes after a chemical weapons attack has already occurred.

These include an Organisation for the Prohibition of Chemical Weapons (OPCW) investigation to establish facts and responsibility and the use of "measures not involving the use of armed force" under article 41 of the UN Charter.

The British PM did not exhausted these steps before committing the UK to missile strikes.

Finally, May claims that the use of force is necessary to enforce the norm against the use of chemical weapons.

Those norms are embodied in the Chemical Weapons Convention, which provides that when a state is in noncompliance, the conference "may restrict or suspend the state party's rights and privileges under this convention until it undertakes the necessary action to conform with its obligations."

When a violation causes "serious damage to the object and purposes" of the convention, the conference "may recommend collective measures to states parties in conformity with international law."

Crucially, the convention establishes a binding obligation to bring matters of "gravity" including facts and evidence to the UN General Assembly and the Security Council.

In other words, the British prime minister's action is directly contrary to the very international norm she is referencing to justify military action.

When a state's action is in serious breach of the convention – a threshold met by Assad's use of chemical weapons last week – the mandated solution is a recommendation for collective measures.

Evidence establishing the violation and any conclusions that flow from such evidence has to be brought before the Security Council for action.

The international norm that prevents the use of chemical weapons does not specifically authorise states to conduct military strikes to enforce it.

The precise language employed in the convention itself unambiguously destroys May's argument.

To be clear, Assad must pay a heavy price for his heinous crimes.

However, distorting international law rules and offering weak justifications for using force are unlikely to earn support from the domestic public or other states to make him pay.

The hard yards of evidence gathering, establishing guilt, and building international support are likely to be much more effective than theatrical one-off strikes.

Dr Sandeep Gopalan is a professor of law at Deakin University, Australia