Tim Dunne is Professor of International Relations at the University of Queensland. He co-edited (with Trine Flockhart) Liberal World Orders, published this month.

In a matter of days, Australia will take up the rotating presidency of the UN Security Council. If the war of words over Syria intensifies, Ambassador Gary Quinlan is going to find himself in the eye of a storm.

Australian foreign policy has been at the forefront of advocating the principle of the responsibility to protect (R2P). It was the publicly stated rationale for Kevin Rudd's diplomatic activism in support of the intervention in Libya in 2011 (when he was minister for foreign affairs), and early indications of Prime Minister Rudd's speech at the Lowy Institute suggest he is once again looking at the crisis through the prism of R2P, evident in his allusions to the poor international response to the ethnic cleansing in Screbrenica and the genocide in Rwanda.

Meanwhile, UK Foreign Secretary William Hague was quoted in a BBC radio interview as saying that military action could be lawful even without a UN Security Council resolution, though he did not provide a rationale for this position.

So what are the legal arguments surrounding foreign intervention in Syria following the chemical weapons attack that occurred in the early hours of 21 August?

The case for military strikes must be predicated on conclusive proof that it was the Assad Government which launched the weapons that killed hundreds of civilians. Being sure of the case is the first rule of fighting a just war. It is a test the US and its allies miserably failed in the Iraq war. 

Reliable evidence is a necessary but not sufficient condition for a military response. Other conditions include having a humanitarian motive for the action; retribution alone will not do. Neither will it be justifiable to target the regime's military assets if this poses an unacceptable risk to civilians.

Even if these conditions are satisfied, the use of force will be unacceptable unless it is justifiable under international law. Again, the Iraq war casts a long shadow over current debates. Back in 2003, the US and its 'coalition of the willing' fought a war of regime change without UN Security Council backing and without an adequate legal justification. George W Bush believed he only need Congressional permission.

Those unipolar days are over. This time a war of regime change fought by air, land, and sea is not on the agenda; neither is adopting the same disdainful view of multilateral institutions that came naturally to America's neocons.

In the days immediately prior to the key UN Security Council debate on Libya, President Obama made it clear that he would not use force even for civilian protection without proper authorisation. Yet it is highly unlikely that the Council will get an agreement for robust action against Syria, meaning the most direct route to international legality will be blocked off by Russia and China, with a few non-permanent members also likely to withhold their support.

This does not mean there are no other possible legal arguments to support the use of force against Syria. The least compelling would be for the US and its allies to claim self-defense despite the fact that the Assad regime has not mounted any direct attack upon them. There might have been a shred of credibility to this claim in late 2012 when Syrian Government shells exploded in Turkey, causing loss of life. A NATO council meeting condemned Syria but did not trigger Article 5 of its constitution whereby the organisation would take collective action to defend one of its members.

An alternative legal argument would be to say that the actions of the Syrian Government have been so shocking that Assad and his ruling party has forfeited the right to rule. Instead, the Syrian National Coalition (SNC) would then be the legitimate representative of the Syrian people, and any military action would therefore be lawful as it would rest on the consent of the state. The weaknesses of this claim include the fracturing within the SNC and the fact that, despite the presence of the SNC at meetings of the League of Arab States, a majority of UN member-states do not recognise it as the legitimate government of Syria.

A more likely basis for a legal defence of Western intervention, outside of the Security Council, would be to argue that proportionate force is justifiable in response to the conscience-shocking events of 21 August. This position would be bolstered by the stark facts of the massive harm that has been perpetrated in two-and-a-half years: over 100,000 civilian deaths, 2 million refugees, and nearer 5 million internally displaced.

In the face of such large-scale atrocities, the 'P3' could claim that diplomacy has failed, and more pertinently from a legal perspective, so has the UN Security Council (a point that the UN Secretary General's envoy Lakhdar Brahimi has previously made). Force would therefore be a last resort justified by the escalating humanitarian emergency.

Could Western militaries find themselves invoking such a right, as they did in 1999 with respect to Milosevic's ethnic cleansing of Kosovo? The convenience of this historical analogy has not been lost on US officials. Getting a prior Security Council resolution proved as elusive then as it has done in the Syria case. But it is worth recalling that US legal counsel at the time of Kosovo was of the view that NATO's action ought not to be interpreted as precedent-setting, since there was a fear that illiberal coalitions could invoke the same argument against Israel or another Western ally.

There is one other aspect of the Kosovo case that is germane to Syria. The conclusion of the Kosovo Commission report recognised what Foreign Secretary Hague is implying, namely, that 'a 2/3 or better majority' could supersede the right of veto where there is a clear and present humanitarian necessity.

If the P3 decide to push for a vote on a draft resolution authorising force with the aim of portraying any veto-wielding members as being irresponsible, then the diplomatic tension inside the Council will be as high as at any point since early 2003. Welcome to the crucible of the Security-Council presidency, Ambassador Quinlan.