British-born historian Robert Conquest, whose influential works on Soviet history shed light on the terror during the Stalin era, has died. He was 98.

Conquest's wife, Elizabeth Neece, said he died Monday of pneumonia in Palo Alto, California.

Conquest was the author of 21 books on Soviet history, politics and international affairs. His "The Great Terror: Stalin's Purge of the Thirties," which documented the purges of dictator Josef Stalin in the 1930s, remains one of the most influential studies of Soviet history.

Published in 1968, the book estimated that under Stalin, 20 million people died in labor camps, executions and famines. It has been translated into more than 20 languages.

"Robert Conquest set the gold standard for careful research, total integrity, and clarity of expression about the real Soviet Union," said George P. Shultz, a former secretary of state and Hoover Institution distinguished fellow.

A renowned historian of Soviet history, politics and foreign policy, he received the Presidential Medal of Freedom in 2005.

Conquest spent 28 years at the Hoover Institution, where he was a senior research fellow. While at Hoover, Conquest again wrote about Stalin's regime in "The Harvest of Sorrow," which depicted the brutal collectivization of Soviet farms under Stalin.

"Once again, he definitively established the colossal scale of Soviet horrors, correctly identified their source in Marxist ideas and practices, and underscored the legions of Western dupes who retailed Soviet lies, from when Stalin was alive and decades thereafter," said Stephen Kotkin, Hoover research fellow and Princeton University historian.

Conquest was also a noted poet and figure in the "Movement" poetry of 1950s England. He was an exhibitioner in modern history at Magdalen College, Oxford, receiving his bachelor's and master's degrees in politics, philosophy and economics.

He served in the British infantry in World War II and in the diplomatic service afterward. He was awarded the Order of the British Empire. In 1996, he was named a Companion of the Order of St. Michael and St. George, the Hoover Institution said.