Alleged chemical weapons attacks against civilians by the Syrian regime look set to trigger a US-led military response. With Russia and China opposed, the UN Security Council will not give its backing, raising questions about the legal basis for military intervention.
The BBC's legal correspondent Clive Coleman says there is a developing legal framework for possible military action on humanitarian grounds. Here, a series of experts give their views on the legal basis for a military strike in the absence of a UN resolution.
There has never been any need for a Security Council resolution approving action to stop, punish or deter a crime against humanity.
Before the UN or League of Nations were established there were well-recognised situations where action was taken against piracy, against slavery.
More recently, we have action taken by Nato to stop ethnic cleansing in Kosovo. That did not require a UN resolution, which Russia would have blocked.
If Russia wants to render [prevent military] action against Syria, for a crime against humanity of using chemical weapons to mass murder its own people, then it must bring a motion of condemnation in the Security Council, as it tried to do with Kosovo.
It failed, getting only three votes, so Nato action in that case is regarded as legitimate. So was the Nato use of force to create safe havens in Iraq in the 1990s.
However, any force used must be proportionate; it must have the objective of deterring future use of poison gas by the Syrian state. That means very limited military action, probably to close air bases, with the threat of more extensive action to come, if there is further use of poison gas by the state.
The world cannot ban chemical weapons and then sit idly by while a state uses them to kill civilians.
However there is a burden of proof on those who wish to use force to establish beyond doubt the culpability of the Syrian state.
This would best be done by bringing the evidence before an international court. But this is perhaps premature and the evidence will have to be brought before the Security Council and if the majority [of members] accepts it then they can go ahead with deterrence by use of force.
That resolves the legal question but there are obviously political and moral questions involved.
The situation in Syria needs to be addressed from the perspective of the UN Charter. If the Security Council decides that the situation represents a breach or threat to international peace and security, they have the mandate to decide to use military means to restore peace and security if non-military means are deemed to be inadequate.
Such decisions need to be agreed by all the permanent members of the UN Security Council. In the current situation it seems unlikely that Russia and China will agree to such measures. Thus, the legality of intervention without the Security Council's clear approval - humanitarian intervention - becomes the question.
Intervention without Security Council approval is not provided for in the UN Charter, but states have, under the Charter, a duty to promote and encourage respect for human rights and fundamental freedoms.
If the Syrian authorities have committed mass killings of its own population, they have committed crimes against humanity. In such situations, the obligation of states to take action is clear on the basis of the Responsibility to Protect, which has been approved by member states of the UN, without having firm legal provisions.
However, these actions need to be exercised in a manner that is likely to minimise the continued human suffering of the population, and a military campaign may not be the appropriate response in such circumstances.
Western intervention could be used as an excuse for the Syrian authorities to commit further atrocities, and thus be the trigger for further suffering.
The use of force by a state in a situation that is a threat to international peace and security, as here, is governed by the United Nations Charter. This requires that the action is approved by the UN Security Council for it to be lawful (unless it is self-defence).
Such approval has been given even where there are divergent views, as with the intervention in Libya in 2011. The lack of such approval was why the armed force in Iraq in 2003 is considered to be illegal.
If there is strong impartial evidence that chemical weapons have been used by the Syrian government then it is a sufficient violation of international law to justify lawful international action.
States may use a range of non-military measures, such as humanitarian assistance to the people affected, and no state should provide military support to a state which has taken these actions against its people.
It has been argued that armed intervention may be possible for humanitarian reasons, as was a justification for intervention in Kosovo in 1999, or as part of each state's responsibility to protect all people who are victims of war crimes, genocide, crimes against humanity, and ethnic cleansing as a gross and systemic human rights violation, which was a key rationale for the UN peacekeeping mission in Darfur in 2006.
While both principles are not yet international law, they indicate a move towards a more human-focused approach to international law-making.
While the lack of action by the Security Council is very frustrating for the many victims of actions by states (both in Syria and around the world), the current international legal system is designed by states for the protection of international peace and security, and to prevent unilateral force.
It provides that armed intervention is unlawful without Security Council approval.
Russia will certainly say that this action [would be] illegal because it definitely won't get the mandate of the Security Council.
But there's a precedent - the military action against Yugoslavia in 1999, which Russia also strongly argued against. And the military action against Iraq in 2003 also didn't have UN approval. Even some of the Western powers were opposed but it didn't prevent the US and Great Britain from acting.
So there are precedents, but Russia will argue that they are cases where law was breached. So the fact that law has been breached twice before doesn't make another illegal action legal.
In addition, the UN inspectors have not yet conducted their investigation, so the US and its allies [are currently] jumping to conclusions and will be on very shaky ground if they opt for military intervention in Syria.
There are always pretexts for a war, but they may be very weak. For example, six months ago the Nato countries were considering the option of using Article 5 of the Nato charter, stating an attack against one member is an attack against all, claiming that Turkey was attacked by Syria.
But to say that Syria - in its state of civil war - could attack Turkey was a very far-fetched argument.
Big countries always find reasons to start wars. But using Nato solidarity would be detrimental to the spirit of Nato.
Certainly the use of chemical weapons is banned under the Geneva Protocol of 1925, so theoretically the US could use that as a reason [for intervention].
But since there is indeed no proof that what's happened in Damascus was not a provocation by the opposition, it would also not be a very good justification - especially since we have the experience of the Iraq war, where Western troops never discovered any weapons of mass destruction.
For a military intervention to really enjoy full legitimacy, a UN Security Council resolution would be needed - and for that the legal basis would be Article 39 and Article 42 of the UN Charter.
If a Security Council resolution is not realistic politically, there are at least three other options.
The first option is to remain in the UN system and invoke a provision that has been used once before - in the 1950s during the Korean crisis - when the Security Council was blocked.
The parties moved to get a resolution from the UN General Assembly. That does not have the same legal weight, but nonetheless it could be something to take into consideration because that would be recognition of a potential operation with [some] political legitimacy - especially if there is a mass vote in favour.
The other options would be outside the UN framework.
One lies under the "Responsibility to Protect" (R2P) concept.
The 1999 Kosovo operation did not receive a UN mandate, but went ahead nonetheless on the basis of the R2P principle. The quest to find a sound legal basis for Kosovo-type interventions [later] led to the codification of R2P in 2005.
The third option is one which would come into force if Syria were deemed to have violated one of the legal obligations of the international order by using chemical weapons, which have been banned under the 1925 Geneva Protocol.
This third option was not used in Iraq in 1988, when Saddam Hussein's regime used chemical weapons against the Kurds. But nonetheless there might be an opportunity today to invoke this specific legal justification on the basis that Syria has violated its commitments under the Protocol.
If there's no UN Security Council authorisation for the use of force then it's very difficult to argue that any use of force would be lawful.
The argument that the UK initially put forward is the closest one could come to a legal basis, namely the doctrine of humanitarian intervention - the idea that force is permissible when there's a pressing humanitarian need.
But this is a view of international law that has been rejected by most states, even the US. The related doctrine of "responsibility to protect" also does not create a legal right for intervention without Security Council approval.
The Syrian situation is different from Kosovo, where the US did use force for humanitarian reasons. In Kosovo the Security Council had not authorised the use of force but had said that Milosevic should do certain things, and so the US said that [their actions] were part of enforcing the Council's will.
In the case of Syria, they've not even been able to get the Security Council to make these demands.
One possible option would be to go through the UN General Assembly.
In the past it has been argued that when the Security Council is blocked, the General Assembly may be able to authorise the use of force. The General Assembly has been active in adopting resolutions regarding Syria so a successful resolution might be possible there.
[However] it would not be a straightforward case of saying that General Assembly approval automatically makes it legal.
I suspect the reasons this option hasn't been used in the recent past are because: (i) it wasn't clear if the Assembly would approve the action and (ii) members of the Security Council do not want the General Assembly to be seen as overriding the Security Council. Even though the UK and the US decry the impasse in the Security Council in this case, they still value [it] having a degree of monopoly.
This situation in Syria is not comparable to Libya because there we had a UN Security Council resolution. It is not even comparable to Iraq in 2003 because, at least according to some, there was also a Security Council resolution allowing for the use of force in that case.
The only recent precedent we have is Kosovo in 1999.
Since there is a lack of Security Council resolutions in this case, and since there is no treaty expressly providing for the use of force, the only legal basis we could have would be customary international law.
In Kosovo, the states belonging to Nato believed they were entitled to use force because there was no alternative for terminating gross and systematic human rights violations. Other states opposed that idea outright and considered it to be an unlawful attack by Nato.
In Syria, however, the issue of chemical weapons may become a critical factor.
Not only the use but also the possession and manufacture of chemical weapons is prohibited. But does this mean that other states are entitled to respond by the use of force if a state uses such prohibited weapons on its territory? The relevant treaties preventing chemical weapons use do not provide for this.
There seems, however, to be a growing tendency in state practice which deems any use of chemical weapons such a gross violation of international law that it allows a limited use of force in order to terminate such use.
But there are doubts as to whether such a use of force - or humanitarian intervention - has already become an established rule of international law. We may now be in a situation where the pendulum may swing in one direction or the other in that debate, without us being able to predict where it will swing.