The ghastly fate of Mr Skripal seems particularly close to home if one lives not many miles away from Salisbury.

There will no doubt continue to be much debate and discussion about various aspects of this case. But certain things are already clear. Following the lamentable episode of WMD in Iraq, very many people have understandably become suspicious of any attribution to a foreign power of responsibility for anything.

But when one is trying to learn from history, there are two equal and opposite dangers. The first is to ignore the significance of a pattern observed in the past. The second, however, is to be so concerned about repeating the mistakes of the past that one makes mistakes in the present.

In this particular case, there is obviously a great deal that is murky. But some things we do know. Mr Skripal had been a Russian intelligence officer who had betrayed the Russian state, and the nerve agent used in the attack is one that has been manufactured in Russia.

As the PM pointed out, there are only two plausible explanations: either the Russian state used the nerve gas or the Russian state lost control of some of its nerve gas stocks to some other freelance operator. Given that the Russian state has not revealed any breach in the security of the stocks, one is driven very clearly to the conclusion that Mr Putin is perfectly happy to be regarded as the author of the attack.

And if that is so, had the means to carry out the attack, and had previously made some rather blood curdling remarks about the likely fate of those who betrayed the Russian state the circumstantial evidence for supposing that this was an authorised act of state becomes compelling.