The law of war contains a hidden foundation. This foundation is species war. In this paper I begin to develop the idea of species war and show how our modern conceptions of the laws of war contain within them the historical and contemporary operation of species war. In focussing upon how legal and moral decisions about "legitimate violence" are made with respect to judgements about the "value" of "life," I show how species war operates as a fundamental, but often forgotten category, of legal and political philosophy.