Prime Minister Winston S. Churchill, President Franklin D. Roosevelt and Premier Josef Stalin at Yalta. Photo / Getty Images
"Close to frozen," is how Russia's ambassador to the UK, Andrei Kelin, recently described relations between London and Moscow.
His frankness raised a few diplomatic eyebrows, but the comment should come as no surprise. Ever since the 1917 Bolshevik Revolution, hostility between the two nations has been the norm - not the exception.
The list of spats is long and impressive. In 1919, Britain sent troops to fight against the Bolsheviks ("typhus-bearing vermin" was Winston Churchill's description of the new Soviet regime). In 1927, diplomatic relations were temporarily severed after Britain raided the offices of Russia's trade delegation in London, searching for missing documents (they weren't found).
There have been defections, abductions and state-sponsored murders on British soil: Georgi Markov in 1978, Alexander Litvinenko in 2006, and the poisoning of Sergei and Yulia Skripal in 2018 - a failed attempt which led to the expulsion of 23 Russian diplomats from the UK.
On the other side of the balance sheet is a brief thaw in relations during the Gorbachev/Yeltsin era, along with the uneasy alliance with Stalin during the Second World War.
This had the outward appearance of being cordial, but it was fragile and deeply schizophrenic, as is revealed in a sensational but little-known file in the National Archives.
The file (CAB 120/691) contains an astonishing blueprint for war against the Soviet Union - one that was to be fought by the Western allies within two months of German surrender.
The story begins at the 1945 Yalta Conference, when Churchill offered a fulsome toast to his Soviet ally. "It is no exaggeration or compliment of a florid kind," he began, "when I say that we regard Marshal Stalin's life as most precious to the hopes and hearts of us all... We have a friend whom we can trust, and I hope he will continue to feel the same about us."
The prime minister had hoped such gushing words would encourage Stalin to support his plans for a democratic post-war Europe.
But Stalin had no intention of relinquishing control over Central and Eastern Europe and was prepared to renege on the agreements struck at Yalta. His duplicity incensed Churchill and led to a spectacular, if private, volte face in his policy towards his erstwhile ally.
At the beginning of May 1945 - just days after the Red Army captured Berlin - Churchill ordered his joint planning staff at the war office to draw up "Operation Unthinkable", a massive ground, air and naval offensive against the Soviet Union.
Unthinkable was conceived of as a military thrust by the Western allies deep into Soviet-occupied territories; its aim was to impose on Russia "the will of the United States and British Empire".
The strategic architect of the offensive was Brigadier Geoffrey Thompson, a former commander of the Royal Artillery with expertise in the terrain of Eastern Europe.
Thompson's task was to think the unthinkable - a surprise attack on Soviet forces within eight weeks of the Armistice. His battle plan envisaged a massive drive towards Berlin and beyond, with British and American divisions pushing the Red Army back to the Oder and Neisse rivers, some 90km east of the German capital.
"The date for the opening of hostilities is 1 July 1945," wrote Thompson. The initial assault was to be followed by a climactic showdown in the countryside around Schneidemühl (now Pila, in northwest Poland).
This was to be an armoured clash on a massive scale, far greater than the Battle of Kursk - the largest tank offensive in history, with 6000 vehicles fighting on the Kursk salient.
Operation Unthinkable was to involve more than 8000 and would utilise American, British, Canadian and Polish forces.
Yet the odds against the Western allies were heavy. The Soviets had 170 divisions available that spring, whereas the Americans and British could muster just 47.
Thompson reckoned defeating the Red Army would require the use of additional forces and he knew exactly where to find them, proposing the rearming of both the Wehrmacht and SS.
This proposition - while explosively controversial - would add another 10 divisions to the Western army, all of them hardened by six years of warfare.
The level of detail in the Operation Unthinkable blueprint is remarkable: it includes tables, charts and maps of the planned offensive. Four annexes list the precise disposition of Soviet and Allied forces, along with proposals for the aerial bombardment of strategic communications and the use of tactical support for ground forces. Allied naval superiority was also to be used to good effect, with an early seizure of the Baltic port of Stettin.
Thompson believed that stopping exports to Russia from the West would cripple the Red Army - in 1945, the Soviet Union was dependent on America for explosives, rubber, aluminium and copper, as well as half of its high-grade aviation spirit.
Yet the Soviet forces were surprisingly versatile. "The Russian Army has developed a capable and experienced High Command and moves on a lighter scale of maintenance than any Western Army and employs bold tactics based largely on disregard for losses in attaining a set objective," warned the Brigadier.
He told Churchill that "we should be staking everything upon one great battle in which we should be facing very heavy odds".
The prime minister's chief military adviser, General Hastings Ismay, was sceptical of the battle plan and his doubt turned to outright horror when he read of the proposal to rearm the Wehrmacht and SS.
Such a policy, he said, was "absolutely impossible for the leaders of democratic countries even to contemplate".
He reminded his military colleagues that the government had spent the past five years telling the British public that the Russians "had done the lion's share of the fighting and endured untold suffering". To attack these erstwhile allies so soon after the end of the war would be "catastrophic" for morale.
Field Marshal Sir Alan Brooke was equally appalled and considered the Brigadier's proposal to be an act of supreme folly.
Writing on behalf of Churchill's chiefs of staff, he said: "Our view is that once hostilities began, it would be beyond our power to win a quick but limited success and we should be committed to a protracted war against heavy odds." He concluded that "the chance of success [is] quite impossible".
Whether or not the Americans were consulted is not recorded, although General Patton is on record as saying that the Allies had a moral obligation to support the countries being swallowed up by Stalin.
If President Roosevelt had lived beyond April 1945, he would have been appalled by the idea of Operation Unthinkable.
Determined to retain a good working relationship with Stalin, he spent his dying months selling his idea of a new global body: the United Nations.
The hostility of Britain's chiefs of staff would kill off Operation Unthinkable, and they officially rejected the plan on June 8 1945.
Churchill regretted it, telling foreign secretary Anthony Eden that if Stalin's territorial ambitions were not dealt a definitive blow "there is very little prospect of preventing a Third World War". He warned that the Red Army would soon be an invincible force.
"At any time that it took their fancy, they could march across the rest of Europe and drive us back into our island," he said.
The chiefs of staff remained unmoved. They wanted nothing to do with Operation Unthinkable and enclosed the plan in a grey government-issue folder marked Russia: Threat to Western Civilisation.
It remains in that folder to this day, each page stamped in red ink with the words "Top Secret".
It serves as a timely reminder that relations between the UK and Russia were not just close to frozen, as they are today, but perilously close to war.
The 29-year-old man was married and had two children.