Climate change is undeniably a global problem, but the situation is especially dire for countries whose territory is comprised entirely or primarily of low-lying land. While geoengineering might offer an opportunity to protect these states, international consensus on the particulars of any geoengineering proposal seems unlikely. To consider the moral complexities created by unilateral deploy- ment of geoengineering technologies, we turn to a moral convention with a rich history of assessing interference in the sovereign affairs of foreign states: the just war tradition. We argue that the just war framework demonstrates that, for these nations, geoengineering offers a justified form of self-defense from an unwar- ranted, albeit unintentional, aggression. This startling result places our own car- bon-emitting activities in a stark new light: in perpetrating climate change, we are, in fact, waging war on the most vulnerable.