Robert Conquest, an Anglo-American historian whose works on the terror and privation under Joseph Stalin made him the pre-eminent Western chronicler of the horrors of Soviet rule, died Monday in Palo Alto, Calif. He was 98 years old.

Mr. Conquest’s master work, “The Great Terror,” was the first detailed account of the Stalinist purges from 1937 to 1939. He estimated that under Stalin, 20 million people perished from famines, Soviet labor camps and executions—a toll that eclipsed that of the Holocaust. Writing at the height...