Born in December 1958, Euskadi Ta Askatasuna (ETA) was dissolved on May, 11th 2018. During its almost sixty years of existence, ETA has undergone phases of military offensive and then a decline in its operational activity. In order to understand the limits of these episodes and their reasons, we have created an original database listing all the actions of the armed group. Its analysis contributes to highlight the strategic developments of the clandestine organisation and makes possible to discuss the conclusions previously drawn from the group’s databases of fatal victims. We will thus consider the consequences of methodological choices on the interpretation the history of a clandestine organisation and of its key moments.