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Sub-regional defence cooperation has increased significantly over the past decade. At the same time, theories of security and defence integration continue to focus primarily on traditional analytical levels, i.e. the system, regional, and unit levels, while comparatively little attention has been devoted to sub-regional forms of cooperation. Against this backdrop, this thesis utilises a qualitative content analysis to investigate the characteristics and driving forces of Nordic defence cooperation. A document-based collection strategy is applied, primarily to documents published by Nordic Defence Cooperation (NORDEFCO) and the Nordic governments. The purpose of this thesis is twofold: to describe how a sub-regional cooperation such as the Nordic cooperation has emerged and developed over time, and to explain the driving forces behind it, specifically within the defence sector. The findings indicate that Nordic cooperation initially developed within the societal and political sectors before gradually expanding into the defence sector. This process was largely characterised by non-binding forms of cooperation, which over time contributed to the emergence of a Nordic security community. With regard to the establishment and development of NORDEFCO, the study suggests that shared identity, common values, and mutual trust constituted the primary foundations for deeper defence cooperation. Constructivist factors related to Nordic identity and security culture appear to have facilitated continued cooperation despite historical differences in formal security alignments among the Nordic states. At the same time, specific defence initiatives and trends can also be explained through liberal institutionalist perspectives emphasising economic rationality, cost-efficiency, and material benefits. More recently, the deteriorating European security environment has further strengthened incentives for defence cooperation, reflecting a neorealist logic in which states intensify security cooperation in response to perceived external threats and shifts in the balance of power. This thesis contributes to developing an under-examined field of sub-regional security and defence cooperation, by a systematic investigation of the sub-regional dynamics of the Nordic cooperation. As such, the thesis opens for further research in this area, either in terms of in-depth analyses of integration within other sub-regional sectors or as comparative studies between different sub-regional defence organisations.