Oceans are increasingly shaped by climate change, biodiversity loss, geopoliticaltensions and maritime crime and insecurity. Climate-smart marine spatialplanning (CSMSP) has emerged as a governance framework to integrateclimate action, conservation, and equity into ocean planning. However,defense institutions–key actors in maritime security and major greenhousegas emitters–remain absent from CSMSP discourse. This paper arguesthat integrating defense into CSMSP offers strategic and climate benefits:minimizing defense-driven offshore wind cancellation and thus acceleratingapproval, safeguarding environmental protection and undersea critical energyinfrastructure, accelerating decarbonization through the military’s greentransition, and addressing the defense emissions gap. Yet, integration carriessignificant risks: power asymmetries, spatial exclusion, ecological harm,and militarization of green energy. To reconcile security imperatives withsustainability, transparency and equity, the paper proposes governancepathways: transparent data sharing, conflict-resolution and co-existenceprotocols, and defense marine zoning. “However, the paper warns that whilegreen defense initiatives have climate benefits, there are reasons to curbour enthusiasm”. Specifically, rising global military spending and the resultantmineral-intensive extractivism to support war-readiness threaten to overshadowthese benefits by locking in carbon-heavy supply chains and amplifying upstreamemissions, environmental degradation, and social disposability. Aligning nationalsecurity with climate security thus requires more than technological greening: itrequires transparent cradle-to-grave emissions, and strategic restraint in defensespending, war-readiness, and material efficiency. Ultimately, integrating defenseinto CSMSP is not merely a technical exercise but a normative challenge that willdetermine whether ocean governance advances sustainability and transparencyor succumbs to securitized emissions and green extractivism.