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.