During the Cold War, Soviet intelligence officers would remark: “Any fool can commit a murder, but it takes an artist to commit a good natural death.” Those who poisoned the Russian double agent Sergei Skripal and his daughter, Yulia, in Salisbury, Wiltshire, on 4 March sought no disguise. This was an unashamedly brazen act intended to humiliate Britain and to spread fear and terror. Why else use a deadly nerve agent than for ostentatious effect?

All the available evidence leads to the Kremlin. The rare military-grade nerve agent used in the attack originated in Russia (under the name Novichok – “newcomer”), and Vladimir Putin’s regime has a long record of state-sponsored assassinations (most likely including the 2006 murder in London of Alexander Litvinenko, the Russian defector and former FSB agent, who died an agonising death after being poisoned with lethal polonium 210). Only Mr Putin’s administration had a plausible motive for killing Mr Skripal – even as it flouted the convention that swapped spies enjoy protected status.

The Russian state TV presenter Kirill Kleymenov later chillingly – and mockingly – observed: “Something is not right there [in England]. Maybe it’s the climate. But in recent years there have been too many strange incidents with a grave outcome. People get hanged, poisoned, they die in helicopter crashes and fall out of windows in industrial quantities.”

After Mr Litvinenko’s murder, Britain’s response was faltering and inadequate. Though a handful of Russian diplomats were expelled, Britain feared alienating a country it valued as a partner in the “war on terror”. As home secretary, Theresa May refused to hold a public inquiry into Mr Litvinenko’s death until July 2014, relenting only after Russia signalled its malign ambitions through the illegal annexation of Crimea. Mr Putin’s government is one of the world’s most noxious: it has unrelentingly supported Syria’s murderous Assad regime, it has sought to subvert democracies through a rolling campaign of cyberwarfare, and it has murdered journalists and political dissidents who dare to expose its crimes.

After yet another calculated act of aggression, Britain’s response must be robust and unyielding. No sovereign state can tolerate the attempted murder of innocent civilians on its soil (38 people in Salisbury were treated by medics and 500 warned to wash their clothes and possessions). In a strong House of Commons statement on 12 March, Mrs May rightly promised a “full range of measures” in response.

These should include the freezing of UK assets held by Mr Putin’s allies and associates, punitive European Union sanctions (existing measures reduced Russian economic growth by a mere 1 per cent last year), and the designation of the Russian government as a state sponsor of terror. For decades, London has been a playground for corrupt oligarchs, gangsters and plutocrats. This can no longer be tolerated.

Yet Britain, as Mr Putin knows, acts from a position of unambiguous weakness. It is preoccupied by Brexit, a policy that has fractured relations with major European allies and that has imposed great strain on Whitehall. The warning by Remain supporters that Mr Putin would relish a Leave vote has been vindicated.

When Britain voted for Brexit in 2016, some argued that improved relations with the United States would be a positive consequence. But the White House is now occupied by Donald Trump, an ignorant president who avoids criticising Russia whenever possible. Following the Salisbury attack, Mr Trump offered no expression of solidarity with Britain. When Rex Tillerson, his secretary of state, unambiguously condemned Russia on 12 March, he was summarily dismissed from his post the next day.

The United Kingdom is not impotent: as the world’s sixth-largest economy and as one of Europe’s major military forces, it has significant resources of hard and soft power. Its security alliances must be nurtured and strengthened. But in a new era of global disorder, as the traditional pillars of its foreign policy crumble, Britain is feeling the chill of isolation.