WASHINGTON — In one of the darkest moments of the Vietnam War, the top American military commander in Saigon activated a plan in 1968 to move nuclear weapons to South Vietnam until he was overruled by President Lyndon B. Johnson, according to recently declassified documents cited in a new history of wartime presidential decisions.

The documents reveal a long-secret set of preparations by the commander, Gen. William C. Westmoreland, to have nuclear weapons at hand should American forces find themselves on the brink of defeat at Khe Sanh, one of the fiercest battles of the war.

With the approval of the American commander in the Pacific, General Westmoreland had put together a secret operation, code-named Fracture Jaw, that included moving nuclear weapons into South Vietnam…