The Gulf of Tonkin incident immediately comes to mind.
The Truth About Tonkin:
Questions about the Gulf of Tonkin incidents have persisted for more than 50 years. But once-classified documents and tapes released in the past several years, combined with previously uncovered facts, make clear that high government officials distorted facts and deceived the American public about events that led to full U.S. involvement in the Vietnam War.
On 2 August 1964, North Vietnamese patrol torpedo boats attacked the USS Maddox (DD-731) while the destroyer was in international waters in the Gulf of Tonkin. There is no doubting that fact.
But what happened in the Gulf during the late hours of 4 August—and the consequential actions taken by U.S. officials in Washington—has been seemingly cloaked in confusion and mystery ever since that night.
https://www.usni.org/magazines/navalhistory/2008-02/truth-about-tonkin
Johnson Lied, 55,000 Died
National Security Archive Electronic Briefing Book No. 132 ^ | August 2004 | John Prado
Posted on 8/22/2006, 8:34:54 AM by Hillary’sMoralVoid
Forward: During this election season, you will certainly hear the mantra “Bush lied”. To counter that, here is a REAL example of how intelligence was distorted, resulting in TENS of THOUSANDS of American lives lost.
“The Gulf of Tonkin Incident, 40 Years Later — Flawed Intelligence and the Decision for War in Vietnam”
Signals Intercepts, Cited at Time, Prove Only August 2nd Battle, Not August 4; purported Second Attack Prompted Congressional Blank Check for War
Johnson-McNamara Tapes Show READINESS TO ESCALATE, EVEN ON SUSPECT INTEL. TOP AIDES KNEW OF MISTAKEN SIGNALS, but Welcomed Justification for Vote (Emphasis added)
Washington, D.C., 4 August 2004 - Forty years ago today, President Johnson and top U.S. officials chose to believe that North Vietnam had just attacked U.S. destroyers in the Gulf of Tonkin, even though the highly classified signals intercepts they cited to each other actually described a naval clash two days earlier (a battle prompted by covert U.S. attacks on North Vietnam), according to the declassified intercepts, Johnson White House tapes, and related documents posted today by the National Security Archive at George Washington University.
Compiled by Archive senior fellow and Vietnam expert John Prados, today’s 40th anniversary electronic briefing book includes Dr. Prados’s detailed analysis of the intercepts - only declassified in 2003 - together with audio files and transcripts of the key Tonkin Gulf conversations between President Johnson and Defense Secretary Robert McNamara. The latter are excerpted from Dr. Prados’s book, The White House Tapes (New York: The New Press, 2003). The posting also contains photographs and charts from the Tonkin Gulf incident courtesy of the U.S. Naval Historical Center, a detailed documentary chronology compiled by the State Department’s Office of the Historian for the Foreign Relations of the United States series, a CIA Special National Intelligence on Vietnam.
https://nsarchive2.gwu.edu/NSAEBB/NSAEBB132/
https://nsarchive2.gwu.edu/NSAEBB/NSAEBB132/press20051201.htm
Thanks Grampa Dave. It is good to see that others know the story.