For most of American history, that office conveyed authority, dignity, and some measure of majesty upon its occupant. The great presidents—Washington, Jefferson, Lincoln, the Roosevelts—came to be viewed not merely as capable executives but as figures of myth: They were heroic, selfless, noble, godlike. Time has a way of burnishing reputations. But as late as the middle of the last century, Americans were inclined to view even incumbent presidents with reverence. Faith in the presidency may have reached its apogee soon after the Second World War. The public generally trusted Harry Truman and Dwight Eisenhower to be honest and well intentioned and to put the interests of the nation above their own.

It is no coincidence that the last president to inspire such trust was also the last president elected before the Vietnam War began in earnest. Kennedy’s charisma, and his military bona fides, encouraged Americans to believe in their young president as he confronted a complicated and dangerous world. His promise, in his inaugural address, that the United States would “pay any price, bear any burden, meet any hardship, support any friend, oppose any foe, to assure the survival and the success of liberty” reinforced Americans’ vision of their country as a muscular force for good around the globe.

As president, Kennedy immediately faced the challenge of how to use that power. He refused to send American troops to secure a pro-Western government in Laos. But after the Bay of Pigs fiasco, and having been bullied by Soviet Premier Nikita Khrushchev at a summit in Vienna, he made a different calculation when it came to the continuing crisis in Vietnam, one influenced by domestic political concerns. Kennedy confided to an aide: “There are just so many concessions that one can make to the Communists in one year and survive politically.” With the Vietcong gathering strength in South Vietnam, he felt he had to act.

If not for his untimely death, Kennedy’s legacy might have been sullied while he was in office. Instead, not until the Pentagon Papers were published did Americans discover that he and his administration had harbored misgivings about the political and military progress in Vietnam but never shared their reservations with the public, even as they steadily increased America’s commitment of special forces and military “advisers.”

In August 1963, disturbed by the authoritarian South Vietnamese President Ngô Đình Diêm’s failure to win over the populace or thwart the Communist insurgency, Kennedy approved a plan to encourage a cabal of dissident generals to overthrow Diêm’s regime. In November, rebel troops seized key installations in Saigon and promised Diêm and his ruthless brother Ngô Đình Nhu safe passage out of the country. As soon as the brothers surrendered, they were murdered by rebel leaders. South Vietnam plunged into chaos, and a bad situation got worse.