He grew up mostly in Princeton, N.J. At a private school he attended in New Hampshire, a lacrosse (yes, lacrosse) teammate was John Kerry. It’s worth mentioning Mr. Kerry, because he was the same sort: well-born and imbued with the identical sense of class duty. Mr. Kerry, as is well known, enlisted in the Navy even before he graduated from Yale in 1966, and insisted he be sent to Vietnam.

Mr. Mueller graduated from Princeton that same year and soon enlisted in the Marines. Like Mr. Kerry, he saw combat in Vietnam . He won a Bronze Star and a Purple Heart. “One of the reasons I went into the Marine Corps was because we lost a very good friend, a Marine in Vietnam, who was a year ahead of me at Princeton,” he said in a 2016 interview. “There were a number of us who felt we should follow his example and at least go into the service. And it flows from there.”

Mr. Kerry is a Democrat, and Mr. Mueller a Republican. But in their social stratum, while Republicans surely outnumbered Democrats, it didn’t matter all that much. You could, in those days, be in either party and still have the same sense of duty and even, unimaginable as it seems today, believe many of the same things. Thus Mr. Mueller could be comfortable spending much of his career in the Department of Justice in one form or another, being named to posts by Democrats and Republicans alike.

And that is the Robert Mueller who did not want to be seen as being part of anything too “political.” As a creature of his generation, his class, the Marines and the Justice Department, being political surely goes against every instinct he has.

But there is another ideal that men like Mr. Mueller and Mr. Kerry were raised to uphold: the willingness to stand up to the dark impulses of the moment. There is a story, first reported by Jane Mayer in The New Yorker, that illustrates the mind-set perfectly. Shortly after the Sept. 11 attacks, in the face of all the hysteria, precious few lawyers were willing to defend suspected terrorists. At a Washington dinner party, guests began to turn on Tom Wilner, a corporate lawyer who was at the time defending a Guantánamo detainee.