THE key question about the nerve agent attack on Sergei Skripal, a former spy, and his daughter Yulia is this: was the Russian state behind it? Only by being convinced of the answer could an appropriate response be orchestrated.

In a statement to the House of Commons yesterday, Prime Minister Theresa May demonstrated unequivocally her assessment that the Russian state was indeed behind the attack. That made taking decisive action in response a more straightforward matter. Accordingly, Mrs May outlined a series of measures following what she called “this appalling attack on our country”: 23 Russian diplomats – identified as undeclared spies – given a week to leave; increased checks on Russian private flights; the freezing of Russian state assets where these could be used against UK residents; no attendance by ministers or the Royal Family at the World Cup in Russia this summer; all high-level bilateral talks suspended.

These measures, given the conviction that Russia was behind the attack, were commensurate and widely welcomed in the Commons and beyond. Whatever the evidence to which Mrs May is privy, the case against Russia took into account the country’s record of “state-sponsored assassination”. Russia’s response, meanwhile, had been “sarcastic” and disdainful, with President Vladimir Putin letting himself down in behaving the way he had.

He was not the only one. An intriguing side-effect of the chemical weapon attack, at least as demonstrated in the Commons, was the way in which Labour leader Jeremy Corbyn found himself outmanoeuvred on the matter, not least (intentionally or no) by his own backbenchers.

An important lesson on unity and opposition might be learned here. Mrs May had not unreasonably called for cross-party unity, and Mr Corbyn initially paid lip service to this. But, misjudging the mood, tone and occasion, he addressed neither the attack nor the question of Russian culpability. Instead, he spoke in woolly generalities of working with the United Nations to build a “consensus against chemical weapons”, as if that were a revolutionary idea.

When he went on to complain about a 25 per cent cut to the budget of the diplomatic service, the Tory benches erupted with indignation, perhaps more at the timing of the complaint than the substance. The net effect was to leave Foreign Secretary Boris Johnson more of a man of judgment than the Labour leader. Stranger still, the Scottish National Party’s Westminster leader, Ian Blackford – jeered earlier at Prime Minister’s Questions for accusing the Tories of an “attack on devolution” – found himself cheered loudly by the same fierce political enemies after backing the PM’s measures, and promising to work constructively with the Government, while holding its actions up to scrutiny as any good opposition should.

Yvette Cooper, Labour chair of the Home Affairs Select Committee, was similarly cheered, as was Liberal Democrat leader Sir Vince Cable. First Minister Nicola Sturgeon also showed herself astute in tweeting her support for the “proportionate but firm response”.

So much for those who judged their response to events well – and those who did not. The measures themselves seem as much as the Government can do, though Mrs May hinted at other actions that could not be publicly mentioned. Not sending the royals to the World Cup is hardly likely to terrify the Russians, though the expulsion of so many diplomats – the biggest action of its kind in 30 years – runs the risk of a belligerent response. That is, of course, worrying, particularly given Mr Putin’s evident lack of responsibility on this matter so far. But it had to be done, and the message that such attacks on British soil were unacceptable had to be sent.