Late in 1967, American officials and military officers pushed an optimistic view of the Vietnam War. Military Assistance Command, Vietnam (MACV) said that the war was being won, and that Communist strength in South Vietnam was declining. Then came the Tet Offensive of 1968. In its broadest and simplest outline, the conventional wisdom about the offensive--that it was a military defeat for the Communists but a political victory for them, because it undermined support for the war in the United States--is correct. But much that has been written about the Tet Offensive has been misleading. Edwin...