In 1917, two traumatic revolutions saw Russia crash out of World War One in a series of events that would transform the country into the Soviet Union.

The idea of overthrowing the Tsarist regime had long been attractive to the vast, marginalised Jewish community in the country, where pogroms killed tens of thousands and forced millions to flee abroad.

Many liked the idea of introducing a constitutional system, perhaps one resembling Britain.

My great-grandfather Gregory Wilenkin and his younger brother Alexander were rare examples of Russian Jews who had the power and influence to push for change. One of them would pay for their attempts with his life.

Gregory was the only Jewish member of the Imperial diplomatic service; Alexander, who was always known as Sasha, was a charismatic lawyer and later a decorated soldier.

The end of the First World War found Gregory safe in the Imperial Embassy in London. But Sasha would be incarcerated and executed.

This is my Russian family history: it has a complexity I was not aware of until the 1980s, when my retired aunt started writing our family history, and it has been unravelling ever since.

I learned that our Wilenkin forbears lived in Tsarskoye Selo — renamed Pushkin after the revolution, a town near St Petersburg that contained the imperial summer palace. They were descended from an old Jewish family that originally came from Minsk and held landed estates for the previous two centuries.

In the early 1900s, Gregory was overseas investigating how Japan, the USA and the UK had modernised, and working with banking circles in the United States in the hope of securing loans for similar developments in Tsarist Russia.

Sasha was a lawyer and legal adviser to the British consulate in Moscow and active in the Kadets, a political party pushing for Russia to become a constitutional monarchy. His work brought him in contact with a range of liberals and socialists — many of whom would take part in the 1917 revolutions. Some would be involved in his death.

Sasha regularly visited London and was an ardent Anglophile who had his suits made in London’s Savile Row. According to records still held today by the tailor Henry Poole & Co, the brothers were regular customers. Sasha even had his Russian wartime military uniform made there.

He was staying with Gregory’s family on the Isle of Wight when war broke out in August 1914. He chose to return to Russia to join his cavalry regiment, the Sumsky Hussars, but as a Jew he was barred from being an officer.

His personal connections meant that a compromise was found — he was made personal messenger to the regimental commander, and could thus live and eat with the officers — and he became the most decorated soldier in his regiment.

In February 1917, when the Tsarist regime was toppled and a provisional government installed, all restrictions on Jewish soldiers were lifted and Sasha was promoted. He eventually became chairman of the Fifth Army’s executive committee, which meant he participated and spoke in the Russian parliament, the Duma.

As a well-known orator, he was useful to the provisional government in keeping soldiers in line during a restive period that included the abortive Bolshevik “July days” putsch.

But that November, when the Bolsheviks seized power in Petrograd, Sasha was driven underground. He joined a movement led by Boris Savinkov, funded by Britain and France and supported by “Ace of Spies”, agent Sidney Riley, in the hope of launching a counter-revolution. Tragically, they were betrayed.

His subsequent letters and photographs gave an impression of a grim incarceration alongside dozens of political prisoners.

“I’m not in a mood for writing now,” he scrawled behind one picture, printed above. “We’ve spent a couple of days in rather low spirits as all those of this photo except the five marked with small crosses have been taken away to be shot.”

The same fate awaited Sasha — at the hands of former military colleagues who were now Bolsheviks. But the execution, which was recorded in the JC in September 1918, appears to have been conducted with a sense of deference.

“Forgive us if we shoot badly, Comrade Wilenkin,” one Cheka secret police member reportedly implored. “My men are doing this for the first time. Today is their first execution.”

The same report has Sasha smiling in return: “Forgive me also, comrades, if I don’t die at once. I am not used to it either. I am dying for their first time.”

Sasha died aged 35, unmarried and without children.

Some of what little we know of him came from the KGB. Following the fall of the Soviet Union, they gave me a treasure trove of information including petitions for clemency from Jewish soldiers’ associations, the Cheka arrest warrant and an execution order signed by Felix Dzerzhinsky, the secret police chief.

I have one memento — one of the pips from his military uniform — and I am proud to be a Wilenkin descendant and related to him.

Had Sasha’s revolution succeeded, the Russian Jewish community could have been known for helping to usher in to the country a constitutional regime and not a Bolshevik one. He was a “hero”, the novelist Alexander Solzhenitsyn wrote. “Here is another Jewish name, so far undeservedly little known, not celebrated as it should be”. The British diplomat Sir Robert Bruce Lockhart went further: “he proved himself a lion of Judah in the war. He was in fact the bravest Jew I have ever met.”

It is a fitting epitaph.