There was no ideal response available to Theresa May, having decided that the Russian state was responsible for the attempted murder of Sergei and Yulia Skripal. One possible motive for the attack was to provoke a diplomatic row with the UK. The Kremlin has a long-standing policy of testing western governments’ readiness to tolerate projections of Russian power overseas. An international spat can also whip up patriotic fervour in President Vladimir Putin’s domestic audience, since he is seeking re-election this weekend. By pushing back hard, the prime minister risks giving Mr Putin the reaction he wants. But treating the incident as anything less than an outrageous aggression looks weak. It signals that the use of a chemical weapon on British streets could somehow be excusable.

Mrs May was right to set out a measured retaliatory response. Some of these were economic, targeting financial assets that might abet Russian espionage. Others focused on that capability more directly, including the expulsion of 23 diplomats, identified as “undeclared intelligence officers”. Mr Putin is unlikely to change his foreign policy as a result of unilateral British action. And, while Nato allies and the EU have offered words of solidarity, there is much uncertainty around the potential for coordinated containment of the Kremlin. The lack of such cohesion – especially when Brexit makes Britain look strategically dislocated – may have emboldened Russia.

Play Video 2:07 'They have just one week to leave': May expels 23 Russian diplomats – video

On the question of Kremlin responsibility, Mrs May supported her belief by citing the pattern of “complete disdain … sarcasm, contempt and defiance” in official Russian responses to British requests for an explanation as to how a nerve agent developed by former Soviet weapons facilities came to Salisbury. The goading manner of Mr Putin’s diplomatic mouthpieces seems designed to confirm that offence was intended, while not quite accepting responsibility. This is the style of a rogue state or, more pertinently, given Russia’s predilection for internet-based subterfuge, a troll state.

The leader of the opposition’s response to the prime minister was dispiriting. Jeremy Corbyn invited Mrs May to acquiesce to Russia’s requests that a sample be sent to Moscow for verification – on the supposition that the Kremlin might then honestly try to match it with its own stores. He sounded too keen to find another explanation for the use of the nerve agent novichok in the attack.

There are many reasons to be wary whenever governments ask for cross-party support. Oppositions have a duty to challenge prime ministers in the most critical circumstances. Nations should not act in haste over such issues. But Mr Corbyn’s reluctance to share Mrs May’s basic analysis of the Salisbury incident made him look eager to exonerate a hostile power. In the coming days the diplomatic clash with Moscow is sure to escalate. There is likely to be a campaign of obfuscation and misinformation directed at British audiences. That is the Kremlin’s well-established modus operandi. When matters of national security come to the fore, governments do not acquire a licence to act without check or criticism.

But it is also vital to keep sight of the facts. Britain has been targeted with a chemical weapon and it is almost certain that there is only one plausible culprit with the means and the motive. The prime minister might not have as many tools for retaliation, unilateral or international, as she would like. But she has judged correctly that the time for equivocation, given the sinister nature of Mr Putin’s regime, is over.