For three decades, various international challenges have shaped European behaviour in the fields of security and defence. The end of the Cold War and the rise of expeditionary warfare have led to several transformations affecting all European states and their defence frameworks. Contemporary challenges, and new threats, have reshaped national political and military agendas. It has encouraged the exploration of innovative approaches to military matters. Europeans have progressively re-nationalised their defence policies. However, in times of crisis, European states have been inclined to cooperate. This tendency towards improving operational effectiveness and optimising defence procurements identifies the major reasons why military cooperation must increase. Therefore, we must acknowledge the critical importance of interoperability between states in this process.