UN resolution on IS 'extraordinary step'
The day after the terrorist atrocities in Paris, a number of key countries, including the US and Russia, met in Vienna.
The assembled delegates committed themselves to work towards a comprehensive ceasefire in Syria by the New Year.
However, even if peace can be made between government and opposition in Syria, the meeting was united in its determination to carry on the fight against so-called Islamic State (IS) in Syria until its conclusion.
The following week the UN Security Council confirmed the IS actions in Syria constituted "a global and unprecedented threat to international peace and security".
Indeed, the Council called upon member states which have the capacity to do so to take "all necessary measures" to redouble and co-ordinate their efforts to eradicate the safe haven established by IS in significant parts of Iraq and Syria.
"All necessary measures" is ordinarily UN code for an authorisation of the use of force.
However, in this instance, the resolution was not adopted under Chapter VII of the UN Charter, which permits the Security Council to authorise enforcement action.
Nevertheless, instead of collective enforcement action, the resolution opens the door towards a broad reading of unilateral self-defence for all states against IS.
Under the traditionally accepted reading of self-defence in international law, a state hasd to demonstrate that it is threatened by an actual or imminent armed attack.
Forcible retaliation after the event, or action against a threat that may or may not materialise in the future, is precluded.
It is difficult to make out a case for imminence in relation to invisible terrorist groups - they can strike at any time without warning.
Now, in an extraordinary determination, and in view of the outrages committed in Sousse, Ankara, Beirut, over the Sinai and in Paris, the Council found that IS has the intention and capacity to launch further attacks on a continuous basis.
Accordingly, all states are deemed to be under imminent threat and self-defence is available without the requirement to make out a further case, permitting immediate action against IS in Iraq and Syria.
Intervention by Invitation
Previously, states had to rely on an invitation by a regional state to act.
The Russian Federation argues that the Assad government in Syria has invited Russian forces to engage IS in Syria.
It is not clear that the beleaguered authorities in Damascus still have the authority to invite in foreign forces.
Moreover, at least until the terrorist attack against the Russian airliner over Egypt, the Russian air attacks appeared to target the opposition to Assad, rather than IS.
Western governments also depend on an invitation, although one issued by Iraq.
Baghdad argues that IS is using Syrian territory as a safe haven and it can only defeat them on home soil if that also happens in Syria.
This requires the support of Iraq's Western allies in accordance with the right of collective self-defence and in its recent resolution, the UN Security Council noted Iraq's claim - lending credence to this argument.
Can states outside the region use force in their own defence, instead of relying on a right derived from Iraq's entitlement to defend itself?
Obviously, this question poses itself after the events of Paris - but it is not a new question.
After the 9/11 attacks on New York and Washington, the UN Security Council confirmed that non-state actors - terrorists - can have the capacity to launch an armed attack of military dimension, triggering the international right of self-defence.
Destroying the World Trade Center's twin towers by means of aircrafts filled with kerosene was substantially the same as attacking them with a pair of missiles.
The US and her allies could therefore track al-Qaeda to Afghanistan.
In 2014, Washington invoked self-defence against Khorasan, a group in Syria close to al-Qaeda.
A year later, the UK acknowledged a drone strike against Reyaad Khan and two others in Syria.
What is Islamic State?
IS is a notoriously violent Islamist group which controls large parts of Syria and Iraq. It has declared its territory a caliphate - a state governed in accordance with Islamic law - under its leader Abu Bakr al-Baghdadi.
What does it want?
IS demands allegiance from all Muslims, rejects national borders and seeks to expand its territory. It follows its own extreme version of Sunni Islam and regards non-believers as deserving of death.
How strong is IS?
IS projects a powerful image, partly through propaganda and sheer brutality, and is the world's richest insurgent group. It has about 30,000 fighters but is facing daily bombing by a US-led multi-national coalition, which has vowed to destroy it.
In both instances, the US and UK governments took great care in making the case that the targets had been involved in actual or foiled armed attacks against the US and UK respectively and that further such attempts would inevitably follow.
Both states reported their actions to the UN Security Council and no significant objections ensued.
France had initially been reluctant to join the US and others in attacking IS in Syria.
However, in September, the French air force launched an attack against a Syrian IS training camp.
Paris invoked a string of past terrorist attacks with an apparent Islamic background, culminating in the attacks against the staff of the satirical magazine Charlie Hebdo.
Common enemy of mankind
France argued that IS constituted a direct threat to French national security and that self-defence was available as a consequence.
This was rather a broad claim, going beyond the position of other Western states, but sadly, the Paris attacks have validated it.
IS has now been branded the common enemy of all mankind and states have a fairly broad licence to attack and defeat the movement.
While this move chimes with the universal opposition to terrorism, there are some risks.
Despite the reference to international law and the UN Charter in the resolution, this broadening of authority has occurred outside the system of peace enforcement administered by the Council under Chapter VII.
The shooting down of a Russian fighter jet on the border with Turkey shortly after the adoption of the resolution suggests that the Security Council has opened up the door for forcible action without putting into place the necessary mechanism to enforce coordination and restraint.
Marc Weller is Professor of International Law in the University of Cambridge and editor of the Oxford University Press Handbook on the Use of Force in International Law which appeared earlier this year.