It was that other P.O.W.-turned senator, the unfathomably brave Jeremiah Denton, who first signaled to United States military intelligence — via blinking out “T-O-R-T-U-R-E” in Morse code during a propaganda video — that our prisoners of war in North Vietnam were being treated in gross violation of the Geneva Convention. But it was John McCain who brought that mistreatment viscerally into American living rooms, through a harrowing hospital-bed interview broadcast by CBS News in the fall of 1967.

Fearful eyes bugging out of his head, Mr. McCain speaks haltingly, in obvious physical pain, while fighting a losing battle to keep his lips from quivering. “I would just like to tell my wife,” he says at the end, barely keeping it together, “I will get well.” The prediction did not inspire confidence.

Mr. McCain, who died Saturday at 81, spent more than a half-century trying to teach us about torture — that it produces faulty intelligence, that “every man has a breaking point,” that military personnel derive a motivational pride from America having higher moral standards than its debased adversaries. “Your last resistance,” he writes in his latest book, “The Restless Wave” (written as usual with Mark Salter), “the one that sticks, the one that makes the victim superior to the torturer, is the belief that were the positions reversed you wouldn’t treat them as they have treated you.”

That lesson is fading from view in 2018, disregarded both by a president who believes that torture “absolutely works,” and by a #resistance cadre of ex-national security officials whose own brazen lies about the practice have yet to put a noticeable dent in either book sales or cable-TV contracts.