It’s been 70 years since Congress has heard testimony about the Kremlin’s influence on America akin to what former acting Attorney General Sally Yates just delivered. You’d have to go back to March 1947, when J. Edgar Hoover charged that Moscow was burrowing into the pillars of the U.S. government – and that President Harry Truman was failing to take the threat seriously.

In secret, the FBI was pursuing cases of the utmost gravity: Spies had penetrated the State Department and the project to build the atomic bomb. In public, Hoover pledged “unrelenting prosecution” of subversives and asked Congress for “public disclosure of the forces that threaten America.”

Judging from Yates’ testimony, the Justice Department and the FBI determined in January that Moscow had a potential agent of influence inside the Oval Office: the national security adviser, retired Lt. General Michael T. Flynn. They had decided that Flynn’s conduct – his repeated lying, his deepening secrecy, his shady finances – were a threat to America. “The national security adviser essentially could be blackmailed by the Russians,” Yates testified.

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It is hard to convey the level of alarm this would raise in the mind of the chief law enforcement officer of the United States – which Yates was, briefly, before President Donald Trump fired her at the end of January – or the fear it would inflict at the FBI. Suppose your boss hired someone whom you suspected might be a made member of the mob.

Worse, if it could be worse, in the days before and after Trump’s inauguration, Flynn was lying to his superiors about the nature of his dialogues with Moscow’s ambassador – and they either didn’t know or didn’t care about the nefariousness of his denials.

Vice President Mike Pence, among others, publicly repeated the lie – Flynn’s insistence that he had not discussed the stiff sanctions Washington slapped on Moscow for its malicious meddling in the 2016 presidential election. The Justice Department and the FBI had rock-solid evidence to the contrary.

The Russian embassy in Washington is bugged by American intelligence, and it has been since Hoover’s heyday. Flynn was on tape. Moreover, the Russians therefore knew he was lying up and down the chain of command. That’s kompromat – Russian for compromising information – and that’s how hostile intelligence services twist arms.

At Yates’ hearing Senator Amy Klobuchar asked bluntly: “If a high-ranking national security official is caught on tape with a foreign official saying one thing in private and then caught in public saying another thing to the vice president, is that material for blackmail?”

“Certainly,” Yates said.

Yates had an increasingly urgent series of talks with the Trump White House counsel, Don McGahn, starting on Jan. 26. During their second in-person meeting he asked her: “Why does it matter to DOJ if one White House official lies to another?” I suspect that question may haunt the Trump White House: national security, intelligence, the power of government secrecy itself are all based on trusting one another.

First, Yates explained to him, it mattered that Flynn “lied to the vice president and others,” because, as a consequence, “the American public had been misled.” Then, she continued, “every time this lie was repeated and the misrepresentations were getting more and more specific… it increased the compromise and, to state the obvious, you don't want your national security adviser compromised with the Russians.”

To make matters worse, she told McGahn, “General Flynn had been interviewed by the FBI” three days earlier, presumably on the subjects of his secret diplomacy and his undisclosed payments from RT, the Russian propaganda outlet subsidized by the Kremlin.

How’d he do? McGahn asked. Yates declined to say. This could not have comforted the White House counsel. “I was intending to let him know that Michael Flynn had a problem on a lot of levels,” Yates testified. As she told the senators, lying to the FBI is punishable by up to five years in prison.

What continues to astonish, although the subject barely was broached at the hearing, is that Flynn served in good standing and apparent high repute at the White House for more than two weeks after all this took place. It took the Washington Post reporting on Flynn’s dialogues with the Russian diplomat – and his bold lies about them – to force him out in February.

The only aspect more appalling is what the president of the United States had to say about all this that week: "General Flynn is a wonderful man. I think he has been treated very, very unfairly by the media, as I call it, the fake media in many cases. And I think it is really a sad thing that he was treated so badly."

The Yates hearing was a foretaste of what promises to be a summer of inquiry in Washington. The foundations of our free republic were attacked last year. Moscow tried to destabilize American democracy. It achieved that aim. Did Americans aid and abet it? Did their commander-in-chief condone it? We’re going to have to depend on the FBI, the press and Congress if we hope to get an answer.

(Tim Weiner is a Pulitzer Prize-winning reporter. His books include "Legacy of Ashes: The History of the CIA.")