The Prime Minister insists that any military intervention in Syria should be legal, proportionate and a deterrent to the future use of such weapons.
He calls the use of chemical weapons "morally indefensible" and warns that, without action, other repressive regimes in future might be emboldened to use such weapons.
No right-minded person would disagree. Those morally monochrome arguments are persuasive but, if the Iraq war demonstrated nothing else, it proved the importance of giving due weight alongside moral considerations to questions of the practicality and legality of military intervention.
For one thing, the key question of evidence has yet to be answered. In Iraq, the British and Americans went to war on a false premise; this time around, there is no doubt an atrocity has been committed and there is strong circumstantial evidence to indicate that the Assad regime is to blame. Conclusive evidence, however, remains elusive, which could undermine the legality of any allied military action.
The Prime Minister insists that any such action must be compliant with international law, but whether it would be on the basis of current evidence is already called into question.
Secondly, there is the matter of what the allies intend to achieve. Labour leader Ed Miliband is right to insist that any action should be aimed at deterring future use of chemical weapons and have clear and achievable goals. The consequences of military action are highly unpredictable, as former defence chiefs have warned. One-off strikes could draw the allies into further action. Their purpose would have to be heavily circumscribed to avoid mission creep.
Thirdly, careful consideration must be given to the consequences of air strikes. They are arguably the least-worst form of military intervention, yet they are far from straightforward. While intended to be surgical, the scalpel sometimes slips, with terrible consequences. In Afghanistan in 2008, a military air strike gone wrong hit a wedding party, killing 20 people, most of them women and children.
In Libya, Colonel Gaddafi's dying regime almost certainly inflated the number of civilian casualties resulting from allied air strikes, for propaganda purposes. Could air strikes against military targets in heavily-populated Damascus succeed without killing innocent people? That is highly questionable.
It goes without saying that the recall of Parliament to debate action in Syria is an absolute necessity. Such a weighty decision cannot be left to the gut feelings of Foreign Secretary William Hague and David Cameron.
No doubt the debate will quickly become polarised: for all those who decry military action as unconscionable folly, citing the quagmires of Iraq and Afghanistan, there are others who point to Kosovo and Libya to argue that intervention can be short-term and limit iniquitous, state-sponsored murder. While each conflict brings its own unique diplomatic, military, political and moral challenges, there are lessons to be learned from other recent wars.
It is essential that Britian does not drift into military conflict on a wave of moral outrage without fully considering these lessons.
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