In Moscow lies a corpse the tsar fears so much his spies still guard the grave. When Donald Trump visits Russia, he should take time out from hobnobbing with the killer in the Kremlin to visit the resting place of Alexei Navalny – just as a steady queue of his supporters did on the first anniversary of his death last Sunday.

Like them, the President of the United States will be watched by an array of CCTV cameras connected to a facial recognition database. In Vladimir Putin's Russia, Big Brother is not a reality TV show: it's reality.

Young Russian men who pay their respects at the grave of Navalny, their country's last great champion of democracy, are in special peril because they will be identified, tracked down and sent to be cannon fodder in Putin's 'Special Military Operation' in Ukraine.

A Russian friend of mine who recently went to Borisovskoye cemetery was astonished to see three young lads of military age bow at the tomb and then give the Kremlin's Big Brother cameras the finger. Resistance to the tsar is not yet dead.

The Ukrainians, despite not having anything like enough troops, guns, tanks, drones and rockets, have done extraordinarily well to hold back the Russian Army. The rouble, beset by Western sanctions, is tanking.

At this critical moment, Trump and his slick-haired Defence Secretary, Pete Hegseth, have been conned by Putin into betraying Ukraine. Trump's criticism last week of President Zelensky as a 'dictator' who started the war confuses the invader with the invaded. Worse, it shows that the man in the White House has misread his opposite number in Russia.

Appeasement never ends well, and understanding why Putin fears Navalny in his grave is a necessary corrective to Trump's 21st-century Neville Chamberlain tribute act.

That Navalny died from poisoning, I have no doubt whatsoever. In the paperback version of my book on Navalny's killing, Murder In The Gulag, published last week, I tell the story of an inconveniently diligent Russian cop, Alexander Varapaev of the Russian Investigative Committee, the country's version of the FBI.

When Convict Alexei Anatolyevich Navalny died two days after Valentine's Day in 2024, Detective Varapaev took custody of the dead man's stuff, placed it in sacks and bags and logged the lot, taking pains to do the job properly.

The detective found a mattress, loo paper, letters, notebooks, pens, bedsheets, pillow, duvet, his pea coat, a bar of soap, two pens, a tube of lip balm, deodorant, nail clippers, two packs of Elastoplast, Tolstoy's War And Peace, a mug, a pack of ibuprofen pills, swabs of vomit and specks of vomit.

The last two items never made it into the detective's final custody log. Fancy that.

Likewise, the detective's first draft of what happened that grim day was as follows: 'On February 16, 2024, convict AA Navalny felt a sharp deterioration of his health condition while in exercise yard No 2 of the EPKT [a Russian prison within a prison] after which he informed the duty officer of the institution, who took the latter out of the premises of the exercise yard to the premises of section No 4 of the EPKT.

'There, convict AA Navalny lay down on the floor and began to complain of sharp pain in the abdominal area; he started reflexive ejection of his stomach contents, had convulsions, and lost consciousness, which was immediately reported to the medical staff of the correctional facility.'

Just the day before, Navalny had been cracking jokes at the expense of a trial judge at the other end of a video call, to the real amusement of his jailers at the Polar Wolf penal colony in the Arctic Circle.

All mention of the pain in the stomach, the vomiting and the convulsions were removed from Detective Varapaev's final report, almost as if someone high up in the Kremlin didn't want people to know that he had had Navalny poisoned. The world owes Russian investigative journalist Roman Dobrokhotov and his news outlet The Insider a debt for digging up the detective's homework before it disappeared.

The decision to have Navalny poisoned was made in principle at a secret meeting in December 2023 between Vladimir Putin and FSB boss Alexander Bortnikov, according to a source who is a former Western intelligence officer.

What about the prisoner swap that had been planned – to exchange Navalny for the usual dung heap of Russian murderers and spies held by the West? Well, Navalny vomiting and convulsing points to the swap being sabotaged by the only person with the muscle to have the opposition leader murdered in the most tightly controlled prison in the whole Russian gulag.

The Cluedo cards are laid out for all to see. Not Colonel Mustard in the conservatory with the lead piping, but President Putin in the gulag with the Novichok.

Trump should be aware that his counterpart in the peace talks about Ukraine is a common murderer. I've met four Russians who were Putin critics: Anna Politkovskaya, Natalya Estemirova, Boris Nemtsov and Alexei Navalny. In order, they were: poisoned then shot; shot; shot; poisoned then poisoned again.

Navalny and Putin had utterly different visions of Russia. Navalny's was forged by what happened to his beloved grandparents when he was nine years old in 1986 and the Chernobyl nuclear power plant blew up, tolling the death knell on the Soviet Union. Navalny's paternal grandparents lived in Zalyssia, a dozen miles or so from the doomed plant.

To begin with, the Soviet authorities lied about the severity of the accident, imposed a six-mile exclusion zone and encouraged locals outside it to keep on planting potatoes. It was only when radiation alarms went off the scale in the West that the Soviets widened the zone to 20 miles and Navalny's grandparents had to flee their cottage, never to return.

Chernobyl scarred Navalny for ever.

The nuclear accident was wholly unnecessary because, as any viewers of the fantastic HBO drama-documentary series Chernobyl will know, there had been a previous accident in 1975 in what was then Leningrad, now St Petersburg, which had exposed a catastrophic design flaw in the plant's reactor. The Soviets made the design flaw a state secret and, hey presto!, the reactor blew up again.

Navalny, who until the accident spent his every summer with his grandparents in Zalyssia, understood there was something rotten about a system that lied to its people.

Putin meanwhile climbed up inside that rotten system and owes everything to it: his worldview was formed as a KGB officer in the 1970s and has not changed much. With the fall of the Soviet Union he simply dropped Communism and replaced it with a far-Right Russian imperialism. How his outlook can be squared with the strategic interests of the United States is a mystery.

It's probably too late for Trump to get it, because he seems lost inside his own myth-making. But the Republican Party which got him into power and the Americans who voted for him should learn from Navalny's tragedy, and what it tells us about Putin's Russia.

Navalny was a natural entrepreneur who was radicalised by the monstrous corruption of Russian business. He started out as a campaigner buying little packets of shares in big companies, allowing him to find out how they operated: 'After buying the shares, I began to regularly read company reports, monitor what was written about them in the Press, and I was horrified.'

His very first target was the T-Rex of Russian business, Gazprom. He discovered that the oil and gas giant was flogging lucrative bits of its business to peculiar hollow companies which were, in fact, controlled by Gazprom bosses or their creatures.

The death threats were not long in coming after Navalny exposed what he knew. A fellow activist and journalist had already had his fingers crushed for writing the wrong kind of articles. Follow Navalny's career for a short while and you begin to understand his extraordinary courage.

He was, of course, no saint. From 2007 onwards, he had a three-year flirtation with the Russian far-Right, including producing two racist videos targeted at the country's Chechen Muslim minority in the south. From 2010, he gave up that dark nonsense but he never properly apologised for it – a failure, I suspect, which led to him not winning the Nobel Peace prize down the track.

Before Navalny led the opposition, Russian politics had been a weird mix of Yeltsin's drunken disco dancing and Putin's stodgy viciousness. He came in like a steam train, with jokes and memes and often very funny YouTube investigations, the targets being 'The Tsar of corruption' – as he described Putin to me – and his unmerry men.

 One of the most absurd abuses Navalny exposed was a Putin crony flying his own wife's pet corgis around the world on a private jet – because, according to one of the dogs' handlers: 'It's not that comfortable in business class.' This while ordinary Russians scratched a living in conditions of dire poverty.

In 2008 Putin had to give up the presidency because a constitutional limit prevented him serving more than two consecutive four-year terms. He switched roles with his gormless midget deputy, Dmitry Medvedev, becoming prime minister; in 2012, the two did a second switch and Putin was back in the Kremlin.

Navalny hit the streets and tens of thousands roared him on when he condemned Putin's political machine as the Party of Crooks and Thieves.

Navalny got banged up, again and again, fraudulently banned from elections; his family, friends and associates were hassled, sex-shamed, poisoned and shot.

One, Roman Borisovich, asked him: 'Aren't you afraid?' He replied: 'What are they going to do? Kill me?'

Well, eventually, we know the answer to that. But Navalny did not let fear stop him from fighting the Kremlin. I first met him in Strasbourg in 2018 where he was appealing to the European Court of Human Rights against not being allowed to run in Russian presidential elections that March.

He was cocky, funny, charismatic.

In Russia that spring, I met one of his supporters who liked to wear a Vladimir Putin mask at opposition rallies and who had been tasered, then stabbed and left for dead in the snow.

I met a Team Navalny videographer who had been beaten black and blue, and his office manager who had been hit over the head with an iron bar by a police agent provocateur.

Moscow in 2018 felt like what I've read about Berlin in 1938.

And then in August 2020 he was poisoned the first time. Do watch the Oscar-winning documentary, Navalny, on how he and his pal, the Bulgarian Sherlock Holmes, Christo Grozev, hoodwinked the FSB into revealing details of its failed operation. His hotel room had been broken into and Novichok applied to the inside of Navalny's underpants. He had only survived, an FSB agent revealed, because the flight on which he was travelling made an emergency landing and an ambulance crew had been called to the runway.

Why Navalny dared to go back to Russia in January 2021, after he recovered from the poisoning in Germany, is a mystery. But part of the answer, given in his wonderful autobiography, Navalny, is his deep Christian faith.

What followed his return was torture; then more poison, this time a fatal dose.

To Donald Trump, I say, read about what happened to Alexei Navalny. And if you think you can trust his poisoner, Vladimir Putin, think again.

• Murder In The Gulag: The Explosive Account of How Putin Poisoned Alexei Navalny is published by Headline Books.