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Ideology: During the 1920s and 1930s, the Soviets were particularly adept at exploiting the ideological beliefs of communists, who were willing to betray their own for the "greater good." Even disillusioned former communists, such as Whitaker Chambers and Elizabeth Bentley, became valuable assets for American intelligence.
Coercion: Intelligence agencies often resort to blackmail and threats to compel individuals to cooperate. The Okhrana, the Tsarist secret police, would round up revolutionaries and threaten to keep their comrades in jail or spread rumors about them unless they agreed to work as agents.