PARIS—One of the oldest practices in the world of espionage is the “need to know” rule. No one in an intelligence agency is given more information than he or she needs to do the job. As a result, defectors rarely know a spy’s name.

“At best they will know his code name,” as British investigative reporter and espionage historian Phillip Knightley wrote in 1988. “But they may be able to pick up clues to his identity, both from the type of information he sends and from gossip within the service. Much counterintelligence work consists in trying to find the person who fits all the clues.”

The Russia investigations of today have suffered from such difficulties. It’s rare that all the clues fit. And as the history of the hunt for Soviet spies in the 1940s and early 1950s suggests, once they go public and the worlds of counterintelligence and politics converge, politics tend to win out.