In the end it came as no surprise, but it delivered a shock all the same: Joe Biden issued a letter on Sunday afternoon, declaring that he will not run for reelection.

There has been nothing like this in American history. Other presidents have decided not to seek a second term, but under very different circumstances.

In March 1968, Lyndon B. Johnson, at the end of a 40-minute televised address, said, in an unexpected stunner, “I shall not seek, and I will not accept, the nomination of my party for another term as your president.” But LBJ took this step in March, at the start, not the end, of the nominating season. Only one primary had taken place, and Johnson won it. The blow was that his main challenger, Sen. Eugene McCarthy, came in a close second—with 42 percent of the vote, to LBJ’s 50 percent.

But Johnson, though not quite 60 years old, was exhausted by the Vietnam War, the massive protests, and poor health; he’d talked with his wife and with aides about dropping out for some time. He died five years later of a heart attack—his fifth.

In 1952, Harry Truman decided not to run for a second term. Crushed by a stagnant economy and a stalemate war in Korea, Truman’s approval rating had sunk to 22 percent in a Gallup poll at the start of the year. Sen. Estes Kefauver challenged him in the New Hampshire primary that March and won—with 55 percent of the vote, over Truman’s 44. (Truman’s bowing out opened up the Democratic contest. Sen. Adlai Stevenson won the nomination at a multiballot convention; he was defeated by Gen. Dwight........

© Slate