The delayed sentencing of former national security adviser Michael Flynn — for lying to investigators about “sensitive matters” discussed with the Russian ambassador to the United States during the Trump presidential transition — leaves unanswered questions about alleged collusion between Flynn and the Russians during the 2016 campaign.

It also evokes parallels with another former national security adviser, Richard Allen. Allen played a leading role in the Anna Chennault affair, a secret plan formed by Richard Nixon’s campaign to collude with the South Vietnamese government during the 1968 presidential campaign and sabotage Vietnam peace talks in Paris to ensure a Nixon victory.

The Chinese-born widow of a U.S. Air Force general, Chennault was the Nixon campaign’s conduit to the government in Saigon during the 1968 campaign through her friendship with Bui Diem, the South Vietnamese ambassador in Washington and Saigon’s representative at the peace talks.

Newly discovered documents show that, as foreign-policy coordinator to the Nixon campaign, Allen was deeply involved with Chennault’s election interference and secret meetings with the South Vietnamese ambassador, which began five months earlier than historians previously thought. President Lyndon B. Johnson discovered Nixon’s double-dealing in the final days of the campaign, but he didn’t go public with what he knew, and the Chennault affair was never officially investigated. Allen denied any involvement, and although Nixon chose Henry Kissinger as national security adviser, Allen later assumed the role in the Reagan White House.

Though most historians trace the beginning of the Nixon campaign’s collusion with the South Vietnamese to a secret meeting with Bui Diem in July 1968, previously unpublished calendar entries from Chennault’s personal papers show an earlier meeting, in February that year, just two weeks after Nixon announced his candidacy in New Hampshire. This previously undiscovered meeting means the collusion, involving senior political and foreign-policy staffers, was part of the campaign from the start, transforming our understanding of the Chennault affair and its role in Nixon’s victory.

According to handwritten entries in her personal calendar, Chennault introduced Diem to leading figures in the Nixon campaign on Feb. 16 at Nixon’s apartment in New York, and “met John Mitchell [for the] first time.”

Mitchell was Nixon’s campaign manager, and the entries note that foreign-policy adviser Allen was also there, along with Nixon’s longtime secretary Rose Mary Woods and close friend Robert Hill, a former ambassador to Mexico. Woods and Mitchell were Chennault’s neighbors in the new Watergate East apartment complex in Washington.

On June 1, Allen officially joined the Nixon campaign as foreign-policy coordinator, where he was the point person for contact with Chennault. Three weeks later, Chennault wrote to Nixon, suggesting he meet her “close friend” Bui Diem. Four days later, she wrote again, offering to arrange a meeting with South Vietnamese President Nguyen Van Thieu on a forthcoming visit to Washington.

“No! No!” is scribbled next to this underlined passage in Allen’s copy of the Chennault letter, linked to a handwritten note “to RN,” shorthand for Nixon. “Allen recommends this not be done for any reason and under no circumstances. Proposal dangerous in the extreme and injurious to our [Vietnam] position — i.e. to US nat. interests.” Next to Chennault’s conclusion — “There is so much going on in Southeast Asia and so few of us really know the truth” — Allen scribbled “A.C. included.”

While a meeting with Thieu was deemed too risky, Allen did discuss a “possible meeting with South Vietnamese Ambassador” with Chennault in early July and summarized their conversation in a memo to Nixon: “Mrs. Chennault has apparently asked [Diem] if he would talk to DC [Nixon]. I explained schedule tight, but possible to check on available time. Meeting would have to be absolute top secret, etc. Initiative is ours — if DC can see him, I am to contact Mrs. Chennault, she will arrange. …This would be a good opportunity to get filled in on events in Paris and other developments.”

Referring to the words “Top secret,” Nixon scribbled: “Should be but I don’t see how — with the S.S. [Secret Service]. If it can be [secret], RN would like to see — if not, could Allen see for RN?”

In 1975, Allen told Nixon speechwriter William Safire that “he thought about it a lot but decided a meeting would be a mistake.” In the aftermath of Watergate, Allen’s denial was perhaps understandable, but Chennault and Diem’s memoirs from the 1980s confirmed that the meeting did take place.

Chennault’s calendar for July 12, 1968, reads: “N.Y. to see Dick Nixon with Amb. Biu [sic] Diem 2:00 PM.” In her memoir, Chennault claims Nixon told Bui Diem she was to be “the sole representative between the Vietnamese government and the Nixon campaign. …Please rely on her from now on as the only contact between myself and your government. If you have any message for me, please give it to Anna and she will relay it to me.”

From then on, Chennault grew closer to Mitchell. “At the height of the campaign, I was on the phone with Mitchell at least once a day,” she later wrote, “much of the conversation consisting of messages I had been asked to relay to him by various people both within and outside the campaign.” She told author Anthony Summers that “in the weeks that followed … Nixon and Mitchell …told her to inform Saigon that were Nixon to become president, South Vietnam would get ‘a better deal.’ …They worked out this deal to win the campaign,” she said.

Nixon’s July meeting with Chennault, Diem and Mitchell evokes parallels with the infamous Trump Tower meeting of June 2016, at which Trump son-in-law Jared Kushner, Donald Trump Jr. and Trump campaign chief Paul Manafort met a Russian delegation.

Chennault’s contact with senior figures in the Nixon campaign continued right up until the election. While researching my book, I recently discovered a tantalizing note on Chennault, handwritten on Allen’s campaign stationery and dated “Saturday — early a.m. before election” — meaning Saturday morning, Nov. 2, 1968, three days before the election.

That same Saturday, an FBI wiretap on the South Vietnamese Embassy picked up a call from Chennault to Diem to advise “she had received a message from her boss (not further identified), which her boss wanted her to give personally to the ambassador. … ’Hold on, we are gonna win. … Please tell your boss to hold on.’”

Allen’s handwritten notes indicate that Chennault had been frantically trying to contact Mitchell through his assistant and Ambassador Hill in New Hampshire since Thursday morning. It was now the Saturday morning before the election, and she was trying to get Allen to help. Within hours, “her boss” (probably Mitchell) would give her the message for Diem. Allen’s notes appear to be the missing link between Chennault’s efforts to contact Mitchell and her infamous message to Diem. They suggest that Allen helped put her in touch with Mitchell, who gave her the message for the South Vietnamese.

Allen has consistently distorted the historical record to minimize his role in the Chennault affair, insisting that Chennault was given no encouragement. But recent disclosures in the notes of H.R. Haldeman and now in the personal papers of Allen, Chennault and Hill, confirm that the Nixon campaign was actively supporting Chennault’s efforts and that she was in close contact with Allen, Hill and Mitchell throughout the campaign.

In the final eight days of the 1968 campaign, thanks to Vietnamese cables intercepted by the NSA and loose talk on Wall Street, Johnson discovered what Nixon was up to and ordered the wiretap on the South Vietnamese Embassy and FBI physical surveillance of Chennault. But Johnson and his Cabinet ultimately decided they couldn’t inject intelligence from classified sources into an election campaign. Allen, Hill and Mitchell were never questioned about the Chennault affair, and historians had to wait until “the ‘X’ envelope” containing Johnson’s file on Chennault was unsealed in 1994 to learn what Johnson knew at the time.

Johnson national security adviser Walt Rostow deposited the sealed file on Chennault at the LBJ Library at the height of the Senate Watergate hearings in 1973. In a covering note, Rostow concluded that the key link between the Chennault affair and Watergate was that in 1968, “they got away with it. Despite considerable press commentary after the election, the matter was never investigated fully,” emboldening the Nixon campaign to approve ever more ambitious schemes in 1972, which would lead to Nixon’s resignation.

In a pre-sentencing memorandum, special counsel Robert S. Mueller III described Michael Flynn’s assistance during 19 interviews with Justice Department attorneys as “substantial.” We still don’t know what this will tell us about the Trump campaign’s possible coordination with the Russian government during the 2016 election, but truth will arrive quicker than the painstaking archival research of the Chennault affair, which 50 years later is still revealing its secrets.

By exposing wrongdoing in 2016, the Mueller investigation can help prevent a repeat in 2020. Nixon was caught in 1968, but as it was not made public, he did not learn his lesson and paid the price with resignation six years later, after winning reelection in 1972.