Indeed, the US action may set a precedent for evolving international norms. In the meantime, it puts the US in a weaker position when it insists that other nations comply with international law.
US Secretary of State Rex Tillerson's claims about the Syrian regime's use of chemical weapons last week may well be substantiated by classified sources. But we should bear in mind the equally certain statements one of his predecessors, Colin Powell, made to the United Nations in 2003 about the existence of weapons of mass destruction in Iraq.
Prime Minister Malcolm Turnbull was quick to applaud the United States' cruise-missile strike on Shayrat military airfield, from which the alleged chemical attacks were said to have been launched. This support was apparently provided without thinking much about Australia's usual commitment to due process, or about the legality of US President Donald Trump's action.
While both US domestic and international law are relevant, I will focus on the international circumstances under which it would have been lawful for the US to use military force.
Australia and the US have ratified the 1945 UN Charter, which recognises two requirements for using force within another country without its consent.
The first is the agreement of the UN Security Council, as happened with NATO's 2011 intervention in Libya to impose a no-fly zone and a ceasefire in its civil war. The Security Council did not agree to the recent US strikes in Syria.
The second is the right of self-defence. However, the US Defence Department said the strike was "intended to deter the regime from using chemical weapons again". This is clearly not self-defence.
There are various other circumstances in which the use of military force in another country can be justified, but they do not seem to apply in this case.
Ongoing US-led coalition airstrikes against Islamic State in Syria (including from Australian air-force planes) use the "collective self-defence of Iraq against Islamic State" justification. In other words, they are acting for Iraq's self-defence – at Iraq's request. If Syria was to use chemical weapons against Iraq or against US forces, the extension of the collective self-defence justification could probably be used to cover US retaliation.
Tillerson also mentioned Syria's breach of the 1993 Chemical Weapons Convention and the 2013 UN Security Council resolution No. 2118 (which orders Syria to destroy its chemical weapons arsenal), which covers the imposition of "measures" in response to any Syrian use of chemical weapons. However, the measures do not include authorisation for the use of force.
"Humanitarian intervention" can take place to protect human rights under chapter VII of the UN Charter but all five permanent members of the Security Councilmust agree. However, unauthorised interventions have take place in response to alleged extreme violations of human rights. In 1999, for example, NATO conducted an air war in Kosovo to stop Serb atrocities and ethnic cleansing.
Security Council authorisation is also required for the use of force under the 2005 World Summit-endorsed "responsibility to protect" framework to prevent genocide, war crimes, ethnic cleansing and crimes against humanity.
The correct procedure in the case of last week's chemical incident at Khan Sheikhoun was for an international team of experts to fully investigate the circumstances and then, if considered warranted, international action could be taken against Syria. Such investigations are slower and perhaps less satisfactory in their outcomes, but nation states should not bypass an agreed legal process just to get quicker or more satisfying results.
What, then, are the possible circumstances behind the Khan Sheikhoun incident?
First, that it was a planned Syrian military action using chemical munitions with high-level authorisation. Second, it was a planned military action using chemical munitions without high-level authorisation. Third, it was a planned military action using chemical munitions that had been inadvertently stored with conventional munitions. Fourth, it was an unauthorised chemical attack organised by lower-level military personnel. Fifth, the chemicals were used by a rogue or opposition group with the likely intention of bringing international pressure to bear on to the Assad regime (the Russian position).
All Syria's chemical stocks were supposed to have been removed and destroyed in 2014 in accordance with the US, Russia and Syria-agreed "framework for elimination of syrian chemical weapons". This means neither Syria nor Russia is likely to own up to any of the scenarios involving continued Syrian military possession of chemical weapons.
Well-informed Lebanon-based journalist Robert Fisk notes that, in the past, villages in which Syrian army officers lived – and in which their families lived – have been gassed by opposition groups. "The Syrians blamed the Turks for giving the gas to Jabhat al-Nusra, the al-Qaeda affiliate in Syria and [Islamic State]. The Russians said earlier gas attacks on Damascus used chemical components shipped via Turkey to Syria from Libya."
The US may have compelling evidence, such as command-level communications, confirming that the strike at Khan Sheikhoun was authorised at the highest levels of the Syrian government. But unless that material is placed in the public domain – thereby revealing the extent of US intelligence penetration – it will not be possible to credibly blame Syrian President Bashar al-Assad for the attack.
While international lawyers seem to agree that the US action violated international law, the reality is that permanent members of the Security Council – like the US – are unpunishable in any conventional or meaningful sense. Indeed, the US action may set a precedent for evolving international norms. In the meantime, it puts the US in a weaker position when it insists that other nations comply with international law.
This article first appeared in Fairfax Media on 15 April, 2017.