A half-century has passed since President Lyndon B. Johnson stunned Americans by announcing, in a televised address on March 31, 1968, that he was drastically reducing the bombing of North Vietnam, appealing to the Hanoi government for negotiations and, most incredible of all, withdrawing from the presidential election that fall. One imagines the stupefied reaction in living rooms all across the country: “Did he just say what I think he said?”

Johnson did what modern American presidents are never supposed to do: refrain from seeking re-election. (Since World War II, only Harry Truman in 1952 has done likewise.) He feared that his health could not withstand four more years, but what really worried him was the Vietnam War and the divisions it had created. The war was not just a threat to his personal legacy; it was a threat to the very foundations of the liberal political order that he cherished so deeply and that had built so many middle-class American dreams.

His viewers didn’t know it, but Johnson had always suspected this moment would come. From his earliest days in office, he repeatedly told his wife, Lady Bird, and aides that he felt trapped on Vietnam, that he would be crucified for whatever he did, that the conflict in far-off Southeast Asia would ultimately be his downfall.

Already in May 1964, a year before he committed the country to large-scale war, Johnson said to his national security adviser, McGeorge Bundy: “I don’t think it’s worth fighting for and I don’t think we can get out. It’s just the biggest damned mess.” A year later, shortly before the first American ground forces set foot in Vietnam, Johnson told Senator Richard Russell of Georgia, the chairman of the Armed Services Committee: “There ain’t no daylight in Vietnam. There’s not a bit.”

Publicly, Johnson projected optimism. But the truth is that he was always a bleak skeptic on Vietnam — skeptical that it could be won, even with American air power and ground troops, especially in view of the weaknesses of the South Vietnamese military and government, and skeptical that the outcome truly mattered to American and Western security. ...