This article analyses the mechanisms of the Stalinist deportations from the Western regions of the Soviet Union, which were first annexed after the Molotov–Ribbentrop Pact was signed in August 1939, and again after the Second World War. In particular, it addresses the bureaucratic articulation, involving various repressive, political and administrative bodies, which helps us to understand both the scale and the character of the process. That bureaucratic articulation, based on processing of files and orders, and obeying a repetitive, mechanical logic, was combined with a quasi-military implementation of the forced displacement once everything had been planned on paper, which enabled the mass scale of these operations. These processes are discussed here because they led to mass action and also because they created specific relationships between the individual and the collective. They are studied through the case of a mass operation in Lithuania, code-named “Spring” by the Soviet authorities, for which preparations began in mid-February 1948 and which was completed over two days in mid-May of the same year. Some 12,000 families (about 40,000 people) were deported to Siberia. This article uses various sources: archival documents, at once extremely precise and rich, which aid our understanding of the mechanisms, are combined with oral sources, the testimonies of people who lived through these deportations. These testimonies were collected as part of a major collective project on all of the Stalinist deportations from Central and Eastern Europe and the Western territories of the Soviet Union between 1939 and 1953.