Operation Marigold

Marigold was an American codename for a failed secret attempt to reach a compromise solution to the Vietnam War, carried out by Polish diplomat Janusz Lewandowski, a member of the International Control Commission, and the Italian ambassador in Saigon, Giovanni D'Orlandi, in collaboration with US ambassador in Saigon Henry Cabot Lodge, Jr. at the end of 1966.
In his 2012 book, James Hershberg argues that had not the USA missed this opportunity, direct talks between the United States and North Vietnam could have begun (roughly a year-and-a-half before they actually did, in Paris, in May 1968), the war (or at least the massive US direct military involvement in it) might have ended sooner, and the number of Americans who perished in the war might not have escalated from 6,250 to over 58,000. 
The suspended bombing of Hanoi by the Americans during the period when Lewandowski's mission was there in late November was interpreted as a subtle hint that the USA was open to talks. Via Lewandowski's mediation, both the North Vietnamese and US governments agreed that a direct meeting would take place between their ambassadors in Warsaw, at which the US Ambassador, John A. Gronouski, would confirm that Washington indeed adhered to the positions (known as the "Ten Points") that Lewandowski had conveyed to Hanoi from Lodge. However the efforts halted when bombing in the Hanoi area resumed (for the first time in more than five months) on the 2 and 4, and again on 13–14 December 1966. US President Lyndon Johnson had authorized the continued strikes despite warnings from the Poles, and then from senior members of his own national security team, that any further attacks on Hanoi might cause Marigold to collapse. After the Poles (evidently reflecting Hanoi's wishes) broke off the initiative on December 14, Johnson belatedly halted bombing within a ten-mile radius of Hanoi's center, but despite Polish urging the North Vietnamese leadership refused to reconsider its decision to retract its agreement for direct contacts. Polish Foreign Minister Adam Rapacki relayed Hanoi's final rejection to Gronouski on December 30, and Marigold was dead.
The episode first leaked into public print in a pair of articles by Robert H. Estabrook in "The Washington Post" in February 1967 and was then the subject of an intense investigation by "Los Angeles Times" reporters David Kraslow and Stuart H. Loory, who published their findings in "The Secret Search for Peace in Vietnam" (New York: Random House, 1968). The American side of the affair first emerged in declassified US included in a lengthy chronology included in the Pentagon Paper's diplomatic volumes, which were not leaked by Daniel Ellsberg but emerged only gradually.