It seems almost beside the point to say that the men I encountered in these sorts of operations were in relatively good spirits. They believed in what they were doing — or, at least, believed what they were doing would win the war. After Tet, the violence and brutality continued, but it came from a different place — frustration, anger, resignation, nihilism. After all that destruction, for the enemy to mount such a widespread, if strategically futile, campaign spoke to the great distance between the generals’ grand pronouncements about “light at the end of the tunnel” and the reality on the ground.

The Tet attacks also coincided with the arrival of a new type of G.I., men who had witnessed or heard about antiwar activism and rising drug use back home, experiences they brought with them to Vietnam. Drug use, in particular, reached epidemic proportions in the years after Tet. The decline in morale intersected with a growing trade in heroin, sold in vials of white powder purveyed by roadside hucksters outside bases.

Violence, threatened or actual, against officers became common too. Not infrequently, officers found grenade pins on their bunks — a warning of the “fragging,” or detonation of a fragmentation grenade, that might befall them for hassling their men, especially about drugs — i.e., for acting like officers.

In the summer of 1971, working on an article for The New York Times Magazine, I visited American bases and walked with troops on jungle patrols along with a photographer, David Terry. My angle was also the eventual title of the piece: “Who Wants to Be the Last American Killed in Vietnam.” Tensions among the troops was like nothing I had ever seen before Tet. Men in the field derided support personnel as “remfs,” or rear-echelon — well, you can guess the rest of the acronym. The letters “F.T.A.,” a similarly crude term aimed at the entire Army, were painted on rocks, etched in barrack walls, scrawled in latrines. Black G.I.s shook fists in black power salutes. Antiwar protest crossed racial lines. G.I.s in a simmering state of resentment flashed the “V” sign — not for “Victory,” but for “Peace.”

Col. Rutland Beard, commander of a brigade of army troops with headquarters on “Freedom Hill,” a promontory near Danang, understood his men. “We should have been out of here two years ago,” he told me in 1969. “Let some other people police up the world.” Those were words you never heard pre-Tet.