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 of the Cold War in 1968, when sources about the Soviet Union were scarce, Mr. Conquest was vilified by leftists who said he exaggerated the number of victims. When the Cold War ended and archives in Moscow were thrown open, his estimates proved high but more accurate than those of his critics.

Mr. Conquest also was a much-decorated writer of light verse and a figure in the “Movement” poetry of 1950s England. He continued to publish into his 90s, applying an unyielding zest to poetry and prose alike.

Born in Malvern, Worcestershire, to a British mother and an American father, he served in World War II and then in Britain’s diplomatic corps before a series of stints at think tanks and universities, largely in the U.S. In recent decades he was affiliated with the Hoover Institution at Stanford University, moving to emeritus status in 2007.

While a spirited combatant in academic debate, Mr. Conquest wrote for a wider audience. “The Great Terror” reached millions of readers and won him a following among leaders including Ronald Reagan. Margaret Thatcher consulted Mr. Conquest on how to deal with the Soviet Union and her former advisers said she trusted him more than any other Soviet expert.

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