This article argues that warfare has been marginalised in theories of nationalism but that in conjunction with nationalism is vital for understanding the rise of nation‐states, the formation of nations and the nature of the international system. It offers a critique of statist approaches, suggests mechanisms through which warfare may sacralise nations and explores different patterns of nation‐state formation as they affect the interstate system. In particular, it emphasises tensions between state and nation‐formation as activated by the fortunes of war and the destabilising effects of waves of imperial dissolution, which are accompanied by patterns of re‐imperialization. It suggests that it is simplistic both to claim that war has led to a transition from empires to nation‐states and that contemporary practices of war‐making have led to a postnational era.