Over the years I’ve gotten rid of most of the embarrassing evidence—the photos of us on Telegraph Avenue giving the clenched fist salute while wreathed in choking teargas; the North Vietnamese flag that hung in my front window all those years; the pistol I bought because we all believed that the FBI was coming for us. But one item from the Sixties I’ve kept—a commemorative comb brought back from Hanoi by Tom Hayden after one of his trips there to support General Vo Nguyen Giap’s shrewd perception that the war would not be won in the jungles of Vietnam but on the streets of America.

The comb is machine-cut out of the metal of a downed U.S. aircraft. It is about five inches long, in the shape of an F-105. There are patches of white paint on the unfinished side. A cockpit and insignia have been stamped on the shinier front. Just above the tee ...