Earth orbital space is a militarily and economically critical arena to the U.S. While space has never been forcibly contested to date, this initial period of coincidental quiescence is coming to an end. A growing number of states are developing the means with which not merely to access and exploit space, but to conduct space warfare as well. The time has thus arrived to address the issue of war in space, driving the need for a conceptual framework with which to approach the defense of U.S. space assets and related military and economic security. This paper focuses on developing a theory of space warfare, as distinct from most other treatises in this field, which focus on space power theory. It accomplishes this via a deductive process, exploring some of the current thinking on military theories in general, and examining their applications to space warfare specifically. The result of this analysis is a set of theoretical principles summarizing and describing the nature and needs of war in space. It is hoped that these principles will prove useful in future to the processes of designing weapon systems as well as planning, deploying, training, and conducting space combat strategies and operations.