In June 1937-8, the POUM leadership was arrested and put on trial to investigate the allegations of high treason and espionage made by publications of the Soviet Union and the affiliated Spanish Communist Party. Many across Europe and the Americas saw the prosecution as a litmus test that would indicate the extent to which Soviet intervention had influenced Spanish politics. This paper examines the trail of POUM as a tool of social control intended to rollback revolutionary forms of justice, a process tied to both internal and external (i.e. international) issues of legitimacy. In a time of revolution, the former Spanish Republican state – Spain’s first experiment with liberal parliamentary democracy – attempted to re-establish centralized authority and display its autonomy through a unique form of judicial statebuilding, and the POUM trial was central to this process.