The Servicio de Información Militar (S.I.M.) was created during the summer of 1937 by the Republican state, who was then fighting Franco’s armies. Since its creation, it has been the subject of a « dark legend » assimilating it to an instrument of the Soviet secret services within Republican Spain. In fact, it is first of all a military police and counterespionage service which has been both an instrument and a witness of the reorganisation of the Popular Army from 1937. It was only one year before the end of the conflict that it took up the civil police services. Its reputation of brutality and arbitrariness emerges from its importance in the control of rearguard, in a more and more militarized Republican side. Its weaknesses and abuses belong more to these, more general, of the fighting Republican state than to any control from the Communist Party of Spain. Blaming it for cases of torture and systematic repression of anarchist and poumist opponents appears then as an anachronistic and confusing approach. The S.I.M.’s dark legend, based on the idea of a Communist Party’s political control, is an illustration of the difficulty in understanding the complexity of the relations between, first, individuals and organisations, and, secondly, the parties and state apparatus.