Syria has not attacked Australia or Iraq, so there is no right of self-defence, writes Professor Ben Saul.
The Abbott government is considering the legality of extending Australia's military campaign against ISIS into Syria. In 2014, the United States informed the United Nations Security Council that attacking ISIS in Syria is lawful in the collective self-defence of Iraq.
On a traditional view of international law as it stood between 1945 and 2001, Australian strikes on Syria on that basis would be illegal.
In 1945 the UN Charter established the modern law on the use of military force. One country may use force in another country's territory in three situations: by consent, in self-defence against an armed attack by another country, or if the Security Council gives permission. Self-defence can also be exercised collectively if the victim asks other countries for help.
Syria has not consented to Australian intervention. Syria has not attacked Australia or Iraq, so there is no right of self-defence. The Security Council has not authorised intervention.
This would have been the correct answer before 2001. The difficulty is that the attacks of September 11, 2001, destabilised the traditional law. There is now controversy about whether self-defence is available against a group acting independently of a government.
After 9/11, the US claimed to exercise self-defence against al-Qaeda in Afghanistan, even though the Afghan government, the Taliban, had not attacked it. Most countries accepted the US claim, suggesting that the law had expanded.
In its letter to the UN last year, the US argued self-defence exists if a foreign government is "unable or unwilling" to suppress a terrorist group in its territory that is attacking another country. The UN Secretary-General sympathised, noting that the US strikes "took place in areas no longer under the effective control" of Syria'.
An obstacle is that even after 9/11, the International Court of Justice insisted in a number of cases that self-defence still only exists to counter attacks by another country. A number of judges demurred, arguing the court should take into account 9/11. Some noted the UN Charter itself does not limit self-defence to government attacks, but that the court itself introduced that limitation. Those judges were, however, few.
The legal debate does not address the question of whether strikes are a good idea.
Since 2001, some other countries have claimed expanded self-defence against terrorists in foreign countries, including Israel against Hezbollah, Turkey against the PKK, and Colombia against FARC.
International law is not static but evolves to express the world's collective preferences. This occurs through examining how countries behave, what they say about their own and others' behaviour, and how global institutions respond.
The law may be close to a tipping point, but there is no consensus the law has changed. The short span of sporadic post-9/11 developments is too uncertain to overturn 70 years of stable law. This is true even if few would object to ISIS' destruction.
In an area as important as the use of force, this is as it should be. International law is not just what the US or Australia says it is. Australia learnt this the hard way in 2003, when it was derided for fancifully arguing that invading Iraq was legal.
While the law has not changed, it may need to change to prevent safe havens for terrorists. If the law did change, self-defence would still be limited by the legal principles of necessity and proportionality. Critically, this means Australia could only strike ISIS targets in Syria that are involved in attacks on Iraq. It would be illegal to strike the many ISIS elements that are purely involved in the Syrian civil war. Such differentiation will be very hard in practice.
The legal debate does not address the question whether strikes are a good idea. Bombing ISIS with six aircraft dispersed across two countries will scarcely make a strategic difference, while aggravating the humanitarian exodus on the ground.
If ISIS are more evil than the Nazis, one wonders why we mobilised a million men and women to fight Hitler but can only stump up six planes against ISIS.
If we are serious about ISIS, we would do better to take in more of its Syrian victims, rather than cannibalising the existing refugee quota and substituting desperate Syrians for desperate refugees from elsewhere. We might also think twice about stripping the citizenship of Australian "terrorists", which casts them adrift to keep killing innocent Syrians and Iraqis – and Australians – overseas.