Nixon secretly sabotaged the peace talks in Oct of '68, having a spy indicate to the South that he would give them a better deal. He was afraid a peace agreement would tip the election to Humphrey. That planted the seeds later on for watergate. His fear in the Pentagon Papers thing was the discovery of that Johnson knew he did that.
It was great theater. Nixon knew Johnson had his plane bugged. Johnson knew he knew he had his plane bugged. Nixon knew Johnson knew he caused the talks to fail. Two guys with no morals playing chicken. Of course after the election LBJ got on his hands and knees in the bedroom and showed Nixon his taping system, telling him he'd want something like that so that the academics don't get to write the damn history.