By providing a new qualitative analysis of policy coherence and integration between energy, security, and defence policies between 2006 and 2023, this book analyzes the impacts of policy interplay on energy transition through the lens of sustainability transitions research, security studies, energy security and geopolitics, and policy studies. The security aspects discussed range from national defence and geopolitics, to questions of energy security, positive security, and just transitions. Findings show that the policy interface around the energy-security nexus has often been incoherent. There is a lack of integration between security aspects, leading to ineffective policies from the perspective of decarbonisation and national security, which is evident in the European energy crisis following the war between Russia and Ukraine. This book is intended for researchers and experts interested in the energy transition and its connections to security and defence policies. This title is also available as Open Access on Cambridge Core.