There are three questions the Western world must answer this weekend. First, can force be used, without the approval of the UN Security Council, to deter Syria from perpetrating further crimes against humanity?
Second, how can culpability for mass murdering 1000 civilians by poison gas be proved against the Assad regime beyond reasonable doubt?
Third, what punishment can NATO and the Arab League mete out to that government, if it is guilty, to deter further use of chemical weapons without causing more civilian casualties or tilting the civil war in Syria in favour of the opposition?
The first question is easy to answer. There is and always has been a right to intervene to stop or deter an ongoing crime against humanity. This was asserted by Oliver Cromwell as long ago as 1655, when he threatened to invade Savoy unless its duke stopped killing Protestants who refused to convert to Catholicism.
Britain exercised its right of humanitarian intervention when it stopped the slave trade by intercepting foreign ships and attacking foreign ports and it led a coalition of the willing to end Ottoman atrocities (and liberate Greece) in 1827. As Theodore Roosevelt put it in his State of the Union address in 1904, there are occasions when "the indignant pity of the civilised world" imposes a duty to intervene "against crimes of peculiar horror".
All this was long before the UN Charter, which did not affect the right of member states to stop an international crime. Idi Amin's mass murder was ended by an invasion unauthorised by the Security Council, as was genocide and mass rape in Bangladesh.
NATO set up safe havens to protect the Kurds in defiance of Saddam Hussein's Iraqi sovereignty, without bothering to endure a Russian and Chinese veto at the Security Council.
Kosovo is a good example of legitimate NATO action to end a crime against humanity - that of Slobodan Milosevic's ethnic cleansing. With the Security Council again pole-axed by the Russian veto, NATO simply began bombing and Russia was forced to move a motion to condemn it, which failed miserably.
If NATO and the Arab League can find a way to punish Assad that does not involve collateral damage, they should get on with it and leave Russia on the back foot, without a Security Council resolution to condemn the NATO action. International law does not prevent action to stop international crime.
But, in answer to the second question, there must be proof beyond reasonable doubt that President Bashar al-Assad's forces were responsible for such a crime. The British Attorney-General talks of "convincing evidence generally accepted by the international community", but this is not sufficient. The Security Council itself is a hopeless tribunal for deciding guilt: Colin Powell deceived the council (and, it seems, himself) with his "evidence" for Saddam's weapons of mass destruction.
The world is sick of "dodgy dossiers" and does not believe MI6 and the CIA without proof. What is required is a decision by a panel of independent international judges convened by the UN Secretary-General to decide on Syrian government culpability.
It was the vexed problem of proving Assad's guilty that caused the British parliament on Thursday to refuse to be party to any use of force at this stage. British intelligence argued that Assad's guilt was very likely but not proved beyond reasonable doubt. That conclusive proof may come with the UN weapons inspectors' report in a few weeks if they can tie or trace the chemicals to a delivery system available only to government forces. So long as chemical weapons are not used in the mean time, why not wait?
International law requires that any intervention to stop crimes against humanity must be strictly limited and "proportionate" to the objective, that is, appropriate to and logically connected with its achievement and not such as might promote regime change or the death of more civilians.
This is easy to state, as a legal proposition, but may in logistical terms be impossible to achieve. (Even an attempt to destroy a chemical weapons dump may be disastrous, if the poison gas is released into the atmosphere.) How many military bases are to be attacked? Is NATO going to destroy $1 billion of military equipment and threaten Assad that if chemical weapons are used again it will destroy $25bn worth? It is these imponderables that international law cannot resolve.
The fundamental rules of civilised humanity now include a prohibition on the use of poison gas against civilians - a prohibition barbarically breached by Saddam in 1988 when he gassed 7000 Kurds at Halabja, only to be rewarded the next year by a US trade mission led by Donald Rumsfeld.
If we are serious about banning this horrific war crime, then the ban must be enforced. That is the duty of the Security Council, under Chapter VII of the UN Charter. But it is stymied because Russia irresponsibly uses its "Great Power" veto to protect its investment in Assad's military rule. The responsibility to protect Syria's civilians against becoming victims of crimes by their own state thus devolves upon regional organisations like NATO and the Arab League.
That Assad and his generals are war criminals has been clear ever since they shot the first 1000 or so peaceful protesters before the civil war started. Courageous protesters then held up banners demanding "Assad to the Hague". But the Security Council turned its back on them; not one of its members bothered to suggest that the situation in Syria should be referred to the prosecutor of the International Criminal Court.
Two years on, the civil war has claimed more than 100,000 victims and has no end in sight. Diplomacy and UN negotiators from Kofi Annan to Lakhdar Brahimi have all failed.
NATO strikes to punish chemical weapons use will not end this war and may have unforeseen consequences. But at least such action will create the precedent that should have been set at Halabja, providing a basis for deterring dictators not only from using chemical weapons but from stockpiling them in the first place.