Following years of defence cuts, European defence budgets have been increasing since 2014. Developing the necessary capabilities for effectively balancing the threats to European interests will not only require spending more on defence, but also closer cooperation between the European states. Especially the small European states will need to cooperate with partners and invest in specialised capabilities to be capable of making valuable contributions to multinational operations and collective defence. The proposed research project aims to map and explain the varying degrees to which small European states have pooled their military resources and specialised their forces. The first stage of the research will start with compiling an overview of the military investment decisions of ten small European states to assess the degree to which they have specialised their armed forces and participated in defence cooperation initiatives. Subsequently, the project aims to assess whether the varying levels of specialisation and cooperation are linked to the traditional small state dilemma between autonomy and influence. This will be accomplished by applying Qualitative Content Analysis to official strategic documents. The second stage of the project aims to explain the diverging levels of specialisation and cooperation. This stage of the project will build on a state-of-the-art mixed methods design that combines two set-theoretic methods: QCA and process tracing.