Kremlin agents have deeply penetrated the nation’s capital. Spies for Moscow have burrowed into Congress, into every cabinet department and virtually every agency in Washington, sometimes at the top level. The White House itself is compromised. Meanwhile, media friends of the Kremlin ridicule the notion that Moscow spies on us, mocking those who try to point out the problem.

The year is 1946.

To anyone versed in espionage history, there’s not much that’s particularly new about Donald Trump and his problems with Russia. While our current president is unprecedented in his public kowtowing to the Kremlin, as Trump recently did in Helsinki, the rest of the messy matter that’s currently under investigation by Robert Mueller and his prosecutors would look remarkably familiar to anyone who witnessed the beginning of the last Cold War.

The sides have switched, but the venom (and the unwillingness of many Americans to avoid seeing inconvenient facts) is the same. In 1946, it was Republicans who angrily denounced Democrats for their too-cozy relationship with Moscow which allowed Kremlin spies to take root in Washington. Now, it’s Democrats saying all of that about Republicans. Both are correct, then and now.

What seemed like a paranoid right-wing fantasy regarding Soviet espionage—this was derided as “seeing Reds under beds” by those who wished to distract from the problem—was borne out by the top-secret VENONA program, run by the National Security Agency in close collaboration with the FBI from 1943 to 1980. Based on access to encrypted Soviet intelligence communications during World War II that were compromised thanks to sloppy security procedures, VENONA gradually revealed the shocking extent of Kremlin espionage in America.

That spy network was like nothing the FBI had ever seen, reaching across the country into countless businesses and campuses, and deep into Washington’s corridors of power. VENONA established that Kremlin agents had access to the U.S. government’s closest-held secrets, including the atomic bomb, which Julius and Ethel Rosenberg, a married couple of committed Reds, betrayed to Moscow. NSA codebreakers similarly revealed that Moscow’s spies included some extraordinarily senior Washington officials, including confidants of the president.

The problem facing the Justice Department was that VENONA was by necessity so secret—its existence was not revealed to the public until the mid-1990s—that it could not be used in court. As a result, spies who were firmly fingered by top-secret intelligence, yet the FBI could not develop admissible evidence against, were never charged with crimes, particularly if they kept their denials straight in interrogations with federal agents.

Although the public didn’t know it at the time, by the early 1950s, VENONA broke the back of the vast Kremlin spy network in America. A few traitors were executed, others went to prison, while many more were surrounded in scandal for decades. Moscow’s espionage operations here would not recover—at least not until the era of Trump, when Kremlin agents seem to have wormed their way back into our power circles in a manner not seen in Washington since the 1940s.

The VENONA experience, therefore, is instructive about our current predicament and offers some clear lessons.

First, the Russian spy network is bigger than you think. VENONA includes references to hundreds of Western agents of the Kremlin, who were usually cited by their codenames only. Only a portion of those spies were firmly identified by the FBI, meaning that hundreds of Americans were spying for Moscow in the 1940s but were never charged with any crime or publicly exposed as traitors. Public discussions of Russian spies represent the mere tip of the espionage iceberg.

Second, espionage is a tough crime to prove, even when the government has VENONA in its back pocket. Although Team Mueller has demonstrated willingness to use classified signals intelligence (SIGINT) to prosecute Russian spies—for instance, the recent indictment of 12 Kremlin operatives from military intelligence or GRU on charges of hacking the Democrats, which includes information obviously derived from SIGINT—it’s not yet clear that they will allow sensitive intelligence to drive the prosecution of Americans too. We may know that answer soon.

Third, Kremlin agents will deny their crimes for as long as they possibly can, evidence be damned. Take the case of Alger Hiss, a senior State Department official and one of the leading GRU spies in 1940s Washington, who vehemently protested his innocence down to his death in 1996, just as VENONA went public. His defenders never wavered either. Nine years after Hiss’ death, the NSA released the details of the VENONA intercept that fingered Hiss as a traitor, leaving no doubt about his guilt.

Mort Sobell, one of the members of the Rosenberg spy-ring that betrayed the atomic bomb to Moscow, served 18 years in federal prison and protested his innocence even after VENONA went public. Eventually, however, Sobell’s denials weakened, and in 2008, he admitted that he had spied for Moscow. Just last month, at the remarkable age of 101, Sobell went further, conceding that by secretly serving Stalin he “bet on the wrong horse,” while remaining unrepentant about his crimes.

Fourth, the risk of charlatanry is real and dangerous. Although he was unaware of VENONA, Republican Senator Joseph McCarthy made his name by denouncing shadowy legions of Red spies in our country. By carelessly accusing innocent Americans of betrayal, the notorious Tailgunner Joe did Moscow a great service, since his smears distracted from the real Red spy network in the West, while tarring legitimate counterintelligence work with extremism and chicanery. More than a few U.S. counterspies have wondered what the boozy senator’s real motivations were, and Democrats today will wind up creating their own McCarthys if they’re not careful about distinguishing counterintelligence fact from hothouse spy-mania.

To sum up, the current Kremlin espionage offensive against the West bears uncanny resemblance to the late 1940s. If our Intelligence Community possesses something resembling VENONA 2.0—and, as I’ve reported, there’s good reason to think the IC does—Team Trump is facing a fate resembling that of Moscow’s spies 70 years ago. They will deny until they can deny no more, right until the unpleasant truth finally comes out.

The NSA and the FBI today possess two key advantages compared to the 1940s.

What motivated most Kremlin spies then was ideology, often fanatically held. Ethel Rosenberg went to the electric chair, leaving her sons orphaned, rather than betray the Communist cause. It’s impossible to imagine any members of Trump’s seedy and greedy retinue taking a bullet for the president when even his own longtime attorney, who so recently voiced a willingness to take that bullet, has flipped and gives every sign of cooperating with the Mueller inquiry.

Then, there’s the fact that Moscow’s secret helpers today just aren’t that bright. While the Red cause attracted talent in its heyday—among those fingered by VENONA were some of our best and brightest, including rising-star bureaucrats and scientific prodigies—those secretly working for the Kremlin now are frankly fourth-rate, people for whom Trump’s coming to Washington represented their sole, unlikely avenue to power circles. As clandestine operatives, they are much less cunning than the traitors of the last Cold War. Indeed, many of them can barely be bothered to hide their spy-tracks.

In the months ahead, Congress, and then the public, will learn the findings of Robert Mueller and his investigators regarding the Trump campaign’s actual relationship with Russia on its route to the White House. There will be unpleasant revelations, to be sure, but nobody who is familiar with the VENONA story will be truly surprised by any of it.