Robert Conquest, a historian whose landmark studies of the Stalinist purges and the Ukrainian famine of the 1930s documented the horrors perpetrated by the Soviet regime against its own citizens, died on Monday in Stanford, Calif. He was 98.

His wife, the former Elizabeth Neece, said the cause was pneumonia.

Mr. Conquest, a poet and science-fiction buff, turned to the study of the Soviet Union in the mid-1950s out of dissatisfaction with the quality of analysis he saw at the British Foreign Office, where he worked after World War II in the Information Research Department, a semi-secret office responsible for combating Soviet propaganda.

“The ambassadors varied between people who were interested in politics and people who were interested in music,” he told The Guardian in 2003. “I wanted to study the evolutions at the top in Soviet Russia.”

As one of the Movement poets of the 1950s, a group that included Kingsley Amis, Philip Larkin and Thom Gunn, Mr. Conquest embarked on a research fellowship at the London School of Economics and produced “Power and Politics in the USSR” (1960), a book that established him as a leading Kremlinologist.