In the summer of 1992, Jack Platt, an American, and his Russian best friend, Gennady “Genya” Vasilenko, saw each other for the first time in four years. It was a tense reunion.

Vasilenko had spent that time in a Russian prison and believed that Platt helped put him there.

“Why did you sell me out? Was that the plan from the beginning?” Vasilenko asked, his voice breaking with emotion.

Platt, tears welling in his eyes, answered, “Christ, Genya, you’re the brother I never had. I would never, could never . . .”

Vasilenko didn’t let him finish his sentence, giving his now sobbing friend a bear hug and telling him how much he missed him.

This scene of close male friendship would be touching, save for one bizarre fact. Vasilenko had been a KGB agent; Platt, CIA. And the affection they displayed that day was 100 percent real.

The new book “Best of Enemies: The Last Great Spy Story of the Cold War,” by Gus Russo and Eric Dezenhall (Twelve), out Tuesday, tells the incredible true story of how Vasilenko and Platt were assigned to turn each other and instead became best friends for life.

Platt, born Feb. 18, 1936 in San Antonio, was a whip-smart former Marine known as Cowboy Jack who had a love for guns and a sterling reputation as a dedicated agent.

Vasilenko, born on Dec. 3, 1941, grew up in Siberia during the height of the Cold War and lived “a hard-scrabble Russian existence at its most stereotypical.”

He started drinking “pure alcohol” at age 3, lived without electricity or gas, skated two miles down a frozen river every day to attend school and had a bear cub as his first pet. After joining the KGB, his charm, athleticism and daring led him to be known as the Russian Cowboy.

Vasilenko arrived in Washington, DC, in the spring of 1979.

Posing as a Soviet diplomat, his mission was to recruit CIA and FBI agents as spies, using his athletic ability to meet movers and shakers on DC tennis and volleyball courts.

One of his tennis buddies was a CIA co-worker of Platt’s, and Platt, who knew Vasilenko’s real job from his CIA dossier, was assigned to recruit him. Through his co-worker, Platt arranged to meet Vasilenko and invited him to a Harlem Globetrotters game.

Vasilenko later said he knew almost immediately that Platt, decked out in a cowboy hat and boots and claiming to work at the Pentagon, was either FBI or CIA. Platt later said of that afternoon he knew by halftime how much he liked the Russian agent.

At the game, Platt steered the conversation toward guns, knowing that Vasilenko was a fellow enthusiast, and invited him out to shoot.

The pair went shooting soon after and continued regularly spending time together under the official pretense of trying to recruit each other.

While they didn’t admit their actual jobs to each other for several years, Russo, who interviewed both men together many times, says this was pretty much a formality.

“A few years later they finally confessed to each other, but they almost didn’t have to. They both knew without having to say it,” Russo tells The Post.

“There was no question what was going on.”

This friendship lasted the rest of their lives despite Vasilenko being imprisoned twice in Russia and serving about six years on false charges that he cooperated with his cowboy friend.

Platt retired from the CIA in 1987 but wanted to keep hanging out with Vasilenko, who was then stationed in Guyana. So he made a deal with the agency that he would continue trying to recruit him. He went to Guyana twice a year, where he and Vasilenko shot guns and bar-hopped, and never recruited him.

By 1993, both Platt and Vasilenko, who was fired by the KGB after his first prison stint, had each formed a security consultancy. That year, their companies agreed to a joint venture, and the pair worked together, sharing contacts and making deals, until Russia-US relations hardened in 2001.

Vasilenko moved to the US in 2010, after his second prison stint, and bought a house near Platt, with room on the property for a shooting range.

From then on, the two men saw each other pretty much every day until Platt’s death from esophageal cancer on Jan. 4, 2017, at age 80.

Vasilenko, 77, lives in Virginia. He keeps Platt’s cowboy hat on a shelf in his garage, and has said that when he dies, he wants to be buried next to his old friend.

Before Platt’s death, Dezenhall asked the two about the futility of spy craft, given that so many of their battles end with no winner.

Several weeks later, Platt, having given it some thought, addressed this with Dezenhall.

“We won!” Jack insisted.

“Who’s ‘we’? ” Eric asked.

“Genya and me. We won. How many people get a friend like Genya?”