Political violence, including clandestine violence, unfolds as a process in a dynamic of exchanges between actors involved in concrete conflict situations. In the case of the Basque Country, the birth of the armed group Euskadi Ta Askatasuna (ETA) gave rise to a cycle of violence that lasted sixty years. The group’s actions have provoked a multi-faceted repression that has even included state terrorism. In order to understand the effect of repression on clandestine violence, we have used two original databases (ETA actions, arrests). It appears that, in the specific case of the Basque Country, repression, particularly illegal repression by the Spanish state, had a radicalising effect on the movement, whereas at a latter time legal and judicial repression, inscribed in another socio-historical context, had a deleterious role on the group’s operationality.