This article studies whether the action-reaction model holds on an “embryonic” terrorist group like Galician Resistance (REGA). After pre- senting an overview of REGA’s history, structure, financing, terrorist campaigns, and the police measures adopted against them, the text empirically contrasts whether deterrence is an efficient measure in reducing an incipient terrorist group’s actions. Our results show that deterrence does in fact reduce the number of attacks when aimed at the group’s periphery. However, it causes a backlash of new attacks when aimed at the group’s core. In addition, we prove that an increase in the number of attacks also causes a reaction by police forces and a higher number of detentions of core members. Our results give some meaningful insights into the design of counter-terrorism strategies aimed against “embryonic” groups.