There was a striking use of language in Theresa May's statement to the House of Commons on the Salisbury nerve agent attack. Pointing an accusatory finger at Moscow, the Prime Minister declared:

“ Mr Speaker, on Wednesday we will consider in detail the response from the Russian State. Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom. And I will come back to this House and set out the full range of measures that we will take in response.

This is quite something. It suggests the government is treating this as far more than a murder attempt on a former spy that happened to take place on UK soil. Unless the Russian ambassador can come up with a convincing alibi (e.g. the terrifying possibility that Russia has lost control of its chemical weapons stock), it will treat the poisoning as a state-sponsored attack on the UK as a whole. This is far stronger language than used, for instance, by John Reid in the wake of Alexander Litvinenko's death, even after the authorities had established it was the result of polonium poisoning.

The PM's statement was interesting for two other reasons. First, she emphasised that Mr Skripal and his daughter were poisoned with 'a military-grade nerve agent of a type developed by Russia'. In other words, this was a kind of military attack rather than a civilian or ordinary criminal one, even if the arms were chemical weapons as opposed to conventional ones. Secondly, she emphasised Nato's 'commitment to collective defence and security'.

So this raises the question: might May invoke Nato’s Article 5 on collective defence? That article, from the original 1949 treaty, states that ‘the Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all’ and that they should work together to ‘restore and maintain the security of the North Atlantic area’. It has only been invoked once in Nato’s history – by the USA after September 11th – so this would be the strongest possible option open to her.

If Theresa May does think that Russia has deliberately used a dangerous nerve-agent, in an attempted killing – seriously injuring a policeman and risking injury or death to hundreds more Salisbury residents – perhaps she views Nato as the right alliance through which to co-ordinate a collective response. For obvious reasons, the EU route may be a more complicated one.

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