Fifty years ago today — on Jan. 30, 1968 — roughly 67,000 troops affiliated with the National Liberation Front, a.k.a. the Viet Cong, launched a massive invasion of South Vietnamese cities on the eve of the New Year’s celebration of Tet. Known thereafter as the Tet Offensive, the maneuver brought the Viet Cong enormous troop losses but fundamentally shook the confidence of many Americans, calling into question assurances by President Lyndon B. Johnson that the United States was turning the corner in Southeast Asia and that the enemy’s resources were nearly spent.

The Tet Offensive quickly produced a plunge in support for the war. A Gallup poll from March of that year showed that the portion of Americans describing themselves as “hawks” dropped from 60 percent to 41 percent, while the portion calling themselves “doves” rose from 24 percent to 42 percent. More fundamentally, the overwhelming show of force by the NLF widened the already yawning “credibility gap” and set the groundwork for a decades-long erosion of public trust in government and public institutions.

In this sense, President Trump, whose White House brings mendacity to new and unparalleled heights, is only the most extreme embodiment of a pattern that began a half-century ago.

It all began with a lie.

Initially certain that the war in Vietnam could be won in six months — and loath to rally popular support for a war that might deflect attention, will and resources from domestic policy priorities — the Johnson administration downplayed Vietnam at every critical juncture. Amazingly, neither Johnson nor any other senior official announced or acknowledged that the president had committed ground forces to Southeast Asia. Even when reporters observed evidence of this commitment — including the mass arrival of troops in Vietnam — the administration dissembled, insisting that the mission had not changed. “American troops have been sent [to] South Vietnam recently with the mission of protecting key installations there,” the State Department answered casually.

Only in June 1965 did the department’s spokesman acknowledge that the government had committed to lend “combat support to Vietnamese forces.” But when asked when the president had granted the authorization, he answered, “I couldn’t be specific, but it is something that has developed over the past several weeks.”

The White House soon compounded its credibility problems. If Johnson would not acknowledge that he had committed the nation to war, he certainly could not disclose its cost. So in 1965, he buried Vietnam expenditures — which totaled roughly $5 billion (almost $39 billion in today’s dollars) — in the Pentagon budget. It would shock members of Congress when, six months later, in the middle of the fiscal year, the administration was forced to request a sizable supplementary appropriation to cover the swiftly mounting costs of the war. By 1967, even as Johnson continued to play down the budgetary implications of Vietnam, the war costs approached $26.5 billion for the fiscal year.

So why did the administration lie?

Vietnam posed a grave threat to Johnson’s cherished Great Society — an ambitious slate of programs that included Medicare and Medicaid, assistance to primary and secondary education, Head Start and nutritional assistance for struggling families and students. The administration found itself struggling to pay for both guns and butter. Additionally, wartime spending — funded largely by mounting deficits — created a spiral of inflation that undermined much of the administration’s domestic policy and political standing.

The White House could have financed the war and kept inflation in check had it raised taxes, but asking Congress for a tax increase to finance the war was impossible when the president couldn’t (or wouldn’t) admit what it cost. Such a request would also have provoked a coalition of conservative Republicans and Southern Democrats to demand that Johnson slash domestic spending.

For 2½ years preceding the Tet Offensive, the White House prevaricated, jawboning with labor and industry leaders to keep wages and prices down (without much success) and denying the full extent of the war commitment. In December 1965, Murrey Marder of The Washington Post introduced the term “credibility gap” to readers. It caught fire quickly. “The old C&O canal which runs north from Georgetown is bounded by the Cumberland Gap at one end and the Credibility Gap at the other,” one satirist noted.

Decades before Sean Spicer and Sarah Huckabee Sanders turned White House press briefings into exercises in fiction and fancy, Johnson’s press secretary, Bill Moyers, struggled to conceal the president’s quandary from reporters. It became all the more difficult as Moyers himself grew uncomfortable with the administration’s war policy. Moyers became “less and less successful as presidential press secretary,” White House aide Harry McPherson observed, in no small part because he engaged in frequent “background” discussions in which he attempted to represent the president as also conflicted about Vietnam.

The result was confusion among the press corps: Should reporters believe what the president said, what Moyers voiced publicly or what Moyers shared privately? Or none of the above? By late 1966, some members of the press spoke disparagingly of the “Moyers Gap.”

Whatever inclination Americans had to give their president the benefit of the doubt ended with Tet. Weeks later, CBS’s Walter Cronkite — by some estimations the most trusted man in America — famously repudiated the “optimism of the American leaders” and called into question the “silver linings they find in the darkest clouds.” Johnson is reputed to have said that if he had lost Cronkite, he had lost the country.

In the decade that followed, a series of economic crises and political scandals — most notably Watergate — only widened the credibility gap. By the start of the 1980 election season, trust in public institutions hit record lows. On the eve of LBJ’s electoral victory in 1964, 78 percent of Americans thought that the government could be “trusted to do the right thing” either “always” or “most of the time.” By the eve of Ronald Reagan’s election in 1980, however, that figure had plunged to 26 percent.

Ironically, even though Americans distrusted leaders in both parties with approximate equivalence, Republicans benefited from this trend. After all, if government wasn’t to be trusted, shouldn’t we have less of it? When Johnson was president, 47 percent of voters thought that “people in government waste a lot of money that we pay in taxes”; when Reagan ran the tables in 1980, a resounding 78 percent of respondents voiced this position. Americans also grew more likely to agree that “the best government is the government that governs least.”

Fifty years after the bottom fell out under Johnson, many of his legacy programs — programs that provide health care to the elderly and struggling, education programs and school meals to poor children, civil rights protections to minorities — are on the chopping block in no small part because Americans have been conditioned over time to distrust their leaders.

And as things currently look, the credibility gap is only likely to grow wider.