Deterrence against hybrid threats has gained importance after repeated suspected acts of sabotage targeting critical undersea infrastructure (CUI).
This thesis analyses how the EU and NATO formulate deterrence strategies to protect CUI from hybrid threats in the Baltic Sea region, and to what extent the organisations complement each other. Using a theory-consuming case study design, the study examines policy documents and official communications from 2022 to 2025. Definitions derived from deterrence theory serve as the analytical framework. The findings show that NATO emphasizes military presence and rapid response, representing narrow and immediate deterrence. EU focuses on regulation, resilience, and long-term planning, forming denial, broad and general deterrence. The study identifies a possible tendency for the EU to assume a more prominent role in deterrence efforts. The study contributes to understanding to what extent the EU and NATO, as multinational organisations, use complementary strategies to deter hybrid threats.