Britain’s prime minister makes a sharp and critical speech about Russian attempts to undermine the UK’s institutions. In one sense, not much new there. This is what British prime ministers do, and have done for decades, before, during and after the cold war. This is the UK’s default setting. So when Theresa May made the prime minister’s annual speech to the lord mayor’s banquet this week, a speech traditionally about UK foreign policy, it is not entirely surprising that she used it to mount an attack on Vladimir Putin and his propaganda war against the west. The real surprise might have been if she had done otherwise.

Mrs May pulled no punches. She said the alliances that maintain the global rules-based order must be defended. (It was less clear from her speech in what way, if at all, Brexit contributes to this worthy effort.) But the chief threat to the rules-based order was Russia. Mr Putin’s actions threaten that order, she said, in Crimea, in the Donbass, and through cyber-espionage and disruption. Russia has violated the national airspace of several countries, meddled in elections, hacked the Danish defence ministry and the German Bundestag. In the most striking lines of the speech, Mrs May said: “I have a very simple message for Russia. We know what you are doing.”

Why now? The great issue of the moment for Britain is Brexit. Mrs May could have given a speech about Brexit and still fulfilled the brief of speaking about foreign affairs. She could even have talked about the future of the transatlantic alliance after 12 months of Donald Trump’s presidency. Remember also that Mrs May is not by history and temperament someone who likes to pick a fight with Russia. When she was home secretary she tried to block an inquiry into the killing of Alexander Litvinenko. When she lost that fight she did little to follow up its conclusion that the Russian state, headed by Mr Putin, was responsible for his murder.

There are many things to note about the timing. It allowed Mrs May to restate what she used to describe as her strong and stable leadership. It distracted to some degree from the announcement, on the same day, that the government was making a concession (albeit a questionable one) on the EU withdrawal bill that MPs began debating in detail on Tuesday. It sets a potential dividing line with Jeremy Corbyn who, if prime minister, might be less willing to mount a political attack on Russia, and might come under pressure from pro-Russian supporters not to do so. It is a reminder of how Syria has largely disappeared from the charge sheet against Moscow. It also set some firm constraints for Boris Johnson’s expected visit to Russia in the coming weeks, a trip that has the potential to become a circus.

The speech also marks a significant contrast with Mr Trump’s absurd remarks after a meeting with the Russian leader in Vietnam at the weekend. Mr Trump said Mr Putin had assured him that Russia did not meddle in the US presidential election, adding that he believed the assurances and that Mr Putin felt very insulted by the charges. Mrs May, by refreshing contrast, simply said that this is what Russia does. It is a reminder that, in the real as opposed to the Trump world, it is now just two months before the US is required by its own laws to introduce tougher sanctions against those who cooperate with the oligarchs and companies surrounding Mr Putin. Those sanctions may affect individuals and companies with business in Britain.

In the end, though, these are all secondary issues. The important justification for Mrs May’s speech is simply that it has become increasingly likely that the charges are not paranoid but true, and she is likely to have been briefed to that effect. The evidence has grown of Russia’s sustained efforts to use social media to sway opinion and spread disinformation, and to channel funds to groups and campaigns that cooperated with those aims in elections in France and Germany as well as the US. Russia is not alone among the world’s authoritarian regimes in using these tactics domestically, as a new Freedom House report points out, but it is a global leader in doing so internationally. Mrs May’s claims that Russia seeks to “weaponise information” and plant “fake stories” makes her the latest western leader to highlight the issue, not the first. It is inconceivable that Britain was not a target. Investigations ranging from the Mueller probe in the US to the modest beginnings of an inquiry into the EU referendum campaign in the UK are the first stages in a process of pouring sunshine into a dark world.