“In your world, many times, you ask your young men to stand up and proudly serve their country. In Russia, sometimes we ask our women just to lie down.”

It may sound like the start of an “In Soviet Russia . . .” joke, but that’s Oleg Kalugin, Vladimir Putin’s old boss at the K.G.B., talking. And he’s expertly authenticating one of the most intriguing, and surprisingly historically accurate, products of his old enemy: The Americans, the FX series that wraps up its second season on Wednesday.

It’s not just using sex to snag secrets—as both spy characters on The Americans have done—that makes the show so authentic.

According to former senior K.G.B. and U.S. intelligence officials who spoke to VF Hollywood, The Americans rings surprisingly true and relevant—even today.

The “illegals” are real, and they really did marry each other.

“The Russians have mastered the art of what we call ‘illegals,’ and that’s what this story is about,” says David Major, who was in charge of counter-intelligence for the F.B.I. during the Reagan years and briefed the former president as a National Security Council staff member. He is an expert on Soviet illegals (deep-cover, non-official spies directed by the Kremlin and operating in the U.S. with fake passports). Under his leadership, Major says the U.S. expelled more than 100 Soviet K.G.B. and G.R.U. military intelligence officers from the U.S. The Americans’s Agents Gaad and Beeman represent the roles Major played in real life.

“What’s unique about The Americans is they are really depicting illegals who are staff officers selected to be married to each other,” says Major. “That really happens and happened.”

And there were way more of them than you might think, and for much longer.

“This country was always a paradise for spies,” says Kalugin, who headed the K.G.B.’s foreign counter-intelligence out of Washington, D.C., in the 1960s and 70s, and defected to the U.S. in the 90s. (Major claims there were 531 moles at American facilities at the beginning of the Cold War.) Kalugin worked under diplomatic cover at the Russian Embassy and handled one of the most serious American betrayers in history, John A. Walker Jr., the former Navy warrant officer and mastermind behind a nearly 20-year naval spy ring. But just like the show’s official K.G.B. rezident Arkady Ivanovich is not aware of Phillip and Elizabeth Jennings across the Potomac, Kalugin had no knowledge of or contact with the illegals.

During the Cold War, Dr. Alexander Kouzminov ran illegals in Europe and supported them in the U.S., or as he tells it, “the main enemy—that is how we called the U.S.A. in our agency.” He says he was an officer for the super-secretive Department 12 (biological espionage), within the K.G.B.’s most secretive section, Directorate S (illegal intelligence). “Our illegals established themselves in the West as medical doctors, biologists, physicians, and other specialists, who dealt with the creation and perfection of biological weapons,” he explains. “My duties included training individual illegals and their deployment, keeping them under control. I prepared their tasks, evaluated the information coming from them, worked on the illegals’ covers, supported their everyday life, including providing the finances, and many other things.”

Kouzminov’s illegals prepared for eventual war against the target country on its soil. When Reagan was shot in The Americans, the Jennings couple was on high alert to carry out sabotage and assassinations.

Major says, “What’s unique about the Russians, which they show in The Americans, is that they’re willing to make a commitment with these people for almost a lifetime. One of the illegal men arrested by the F.B.I. in the 2010 ‘Ghost Stories’ operation had started his mission in 1976.”