For a virtually illiterate peasant born in the depths of Siberia in January 1869, Grigori Rasputin had the most extraordinary impact on history. In fact, muses Sir Antony Beevor, the Russian mystic cum sex maniac might be the most significant individual ever to have lived – and his influence reverberates nearly 110 years after his death.

Beevor, the acclaimed chronicler of Stalingrad, has put the monk – womaniser, rapist, drunk and subject of the insanely catchy Boney M pop song – under the microscope in his magnificent new biography and his conclusions are startling. Indeed, without Rasputin’s influence over Russia’s doomed last royal family, he believes the country’s history of that of the wider 20th century would have been wildly different.

“Alexander Kerensky, the leader of the provisional government after Russia’s February Revolution, said that without Rasputin there would have been no Lenin,” he explains. “I think this was absolutely true and it shows how Rasputin had more impact on history than almost any other individual – without him and therefore without Lenin, not only would Russia have been totally different but the whole century with it.”

In a nutshell, the unkempt yet charismatic Rasputin’s powerful hold over the Romanovs, and what was publicly perceived as his sexual seduction of the Tsarina Alexandra, undermined the royal family’s position in society and among supporters – most damagingly the soldiers who were pledged to protect them.

“The idea that Nicholas II was weak or, worst of all, that he was being cuckolded by a peasant and had no control over his wife meant all respect for the Tsar started to collapse,” the historian continues. “And this is why Rasputin had such an effect on history. When it came to the February Revolution in 1917, just after Rasputin’s assassination, there were no officers of the Imperial Life Guard prepared to draw their swords in defence of the regime.”

The Tsar’s abdication was the first of a series of dominos leading to his family’s brutal murder a year later and the rapid rise of Bolshevism in Russia. Had the troops protected the dynasty, it might not have fallen – so no February Revolution and no October Revolution when the Bolsheviks under Lenin swept to power later that year, thus no Russian Civil War, no famine, no communism, no Cold War and perhaps even no Vladimir Putin today. All in all, it’s a remarkably credible thesis and one that, in Beevor’s dramatic retelling, bears scrutiny.

“The collapse of the Russian royal family basically caused this catalogue of destruction, cruelty and famine – the horrors of the Russian Revolution and Civil War, the 20th century split between red and white, Bolshevik and fascist, communist and capitalist,” he says. “It continued all the way through the Second World War, including the Spanish Civil War, and into the Cold War. Today’s split between the autocracies and liberalism goes back to that original split in the second decade of the 20th century.”

So, in Beevor’s view, did Rasputin, whose piercing eyes and vast sexual appetite are a matter of record, and who clearly wasn’t beyond coercion and even rape, seduce the Tsarina?

“No, definitely not,” he shakes his head. “There was a letter from Alexandra where she wrote that she wanted to fall asleep forever on his shoulder which was stolen and circulated in St Petersburg so people assumed they were sleeping together.

“There were even rumours he was sleeping with the Tsar’s four teenage daughters but this again is totally untrue. The problem was that the rumours were as important as fact in the effect that they could have on the wider population.

“Historians in the past have made the mistake of underestimating their impact. In this particular case, the very idea of the Tsar of all Russians – this omnipotent character who everybody regarded with awe – was totally undermined.”

Beevor, 79, who grew up in London and studied at the Royal Military Academy Sandhurst before becoming a cold war tank commander, serving in Germany, has never shied away from uncomfortable truths or big pronouncements.

When our conversation touches on the UK’s current military preparedness, he asks that his views remain off the record. However, without breaking a confidence, like fellow historians Sir Max Hastings and others, he is deeply concerned about the running down of our armed forces and fears we may yet be called to account.

Beevor knows all about battles and how they are won and lost. His 1998 masterpiece, Stalingrad, was a groundbreaking account of the battle that helped turn the tide of the Second World War against Germany. Its follow-up, Berlin: The Downfall 1945, revealed widespread Red Army atrocities and the industrial-scale rape of German women in the closing months of hostilities. Both were huge bestsellers though the latter saw Beevor denounced by Russia for the “slander” of the Red Army. Today he remains at risk of five years in prison just for setting foot in Russia.

Though banned from the archives, the eminent historian whose later books have examined Arnhem, D-Day and Germany’s 1944 Ardennes offensive, shows little sign of slowing down. His new book, three years in the making, is a typically deep and entertaining dive into his subject – equally ideal for a big read or gentle grazing.

Beevor believes the debauched monk continues to fascinate because of his incredible journey from peasant to royal companion – he became spiritual advisor to the Romanovs in 1905 – via a period as a penniless travelling mystic.

“How did he get to this position of being so incredibly influential? From a historical point of view there’s never really been a precedent, we’ve had éminence grise in the past but that’s been, say, a cardinal influencing a king. In this case, it’s an unwashed peasant who was taken up by the Tsar of Russia and treated as almost a member of the family.”

Equally fascinating is Rasputin’s hold on the popular imagination, not least through the Boney M’s 1978 Euro-disco hit, one of the least likely hit records ever by topic and currently enjoying yet another comeback.

“It’s all over the internet at the moment,” chuckles Beevor, a father-of-two grown up children who lives in Kent and London with his wife, writer Artemis Cooper. “It’s extraordinary but it was a funny coincidence just as my book’s coming out. But there’s no doubt, there does seem to be a sort of renewed fascination in Rasputin.”

The fascination, he continues, “is how did somebody who actually looked as scary as Rasputin have such an extraordinary effect on women?”

In answer to his own question, he continues: “He did have an amazing voice, we’ve all sorts of accounts though sadly nobody ever recorded him at the time. But it could be a very calming seductive voice. Above all though, he understood women.

“All the women who fell for him, and even other women who were in two minds about him, would talk about Rasputin as being the only man who ever understood them. He would address a princess in the manner as he would address a maidservant and some women found this refreshing.”

That’s not to say however that he was some sort of protofeminist, blazing a trail through the patriarchal society. Indeed, Beevor is fascinated by Rasputin’s genuine religious faith and completely sex-crazed behaviour.

“Women who could often throw themselves almost to their knees before him, they’d collect items off his plate like fish bones, they would even offer to clip his nails and sew the clippings into their clothes as a sort of talisman,” he says.

“But as a Holy Man, Rasputin was a total contradiction, because while he was genuinely deeply spiritual, he was also deeply lascivious. Women would fall for him and, to begin with, he would be fairly respectful but once they’d fallen for him he treated them in a pretty off-hand manner to put it mildly.”

Rasputin’s sex drive could also manifest itself in abhorrent sexual coercion.

“When the First World War started, he had all sorts of influence with the empress and therefore also with ministers, and he would get people to do things by scribbling a note for them,” continues Beevor. “There were many women who were desperate to get a husband or son or brother out of military service. And these women were in a very vulnerable position because Rasputin basically expected them to take off their clothes and lie down for him.

“And certainly in a number of cases that we know about, he would actually resort to rape. This is somebody who claimed he loved women, and in some ways perhaps he did, but at the same time he simply could not control himself.”

As for the popular image of the mad monk, Beevor is not convinced.

“Some people say it’s a cliché about Rasputin that he had dirty fingernails and smelt and so forth and he certainly scrubbed up as best as he could. But by the end, because of his dissipation and above all consumption of alcohol – it was staggering the number of bottles of Madeira he would drink during the course of a day – he was in a pretty bad way.”

Beevor sounds as if he might admire his latest subject – who was murdered aged 47 in December 1916 by a group of nobles who resented his influence on the Romanovs – just a little. “Well, only because he was such an extraordinary and fascinating character,” he chuckles. “Though I wouldn’t describe that necessarily as admiration. Hitler had an extraordinary ability to move a crowd, but I wouldn’t say my interest in him was admiration.”

• Rasputin And the Downfall of the Romanovs by Antony Beevor (Weidenfeld & Nicolson, £25) is out now