PARIS—Timing is everything in the history of espionage. Geopolitical winds change. Allies become enemies and people once seen as friendly collaborators suddenly are cast as insidious secret agents. Warnings of intrigue previously downplayed or ignored erupt as apocalyptic alarms.

So it was in the late summer of 1945 when a lowly cipher clerk not yet 30 years old, Igor Gouzenko, at the Soviet embassy in Ottawa, Canada, decided that he and his young wife and baby son were going to defect or, as he would later insist, “escape.” In any case, they sure as hell were not going back to Moscow as ordered.

Gouzenko began to collect and hide secret Soviet documents, hoping that they would buy him asylum and protection when he showed them to Western officials—or maybe the press. He wasn’t sure. He knew that one or two other defectors had done such things in the past. In the late 1930s, a former undercover operative in the Netherlands had even published a book. But Gouzenko also knew a great many people in power in the West did not want to know about Soviet espionage and treachery.