Established in 1923, the OGPU replaced the Cheka as Russia’s central organ of the secret police. In 1934, the OGPU would be replaced by the NKVD, which in turn would be replaced by the MGB in 1943 and the KGB in 1954. On the surface, this may seem confusing. But the good news is that unlike political parties, artistic movements, or schools of fashion—which go through such sweeping reinventions—the methodologies and intentions of the secret police never change. So you should feel no need to distinguish one acronym from the next.]
