The treatment of the law of war and neutrality during the interwar era was dominated by two trends. Scepticism was voiced at the relevance and utility of these fields in light of poor compliance during the First World War and the emergence of jus contra bellum. This did not prevent others from engaging with the most pressing issues, including the challenges posed by technological developments (submarines, asphyxiating gases and military aviation); the protection of civilian populations, especially against the risks of aerial bombardment; and the need to expand the application of the law of war in cases of insurrection and civil war. Later events show that the international community did not address these challenges satisfactorily. Initiatives failed due to ‘technological determinism’ and concern for national interest. However, several developments clearly foreshadowed the transition from a law of coexistence to one of co-operation that would come to fruition with the Geneva Conventions of 1949. Indeed, many of the fundamental tenets of modern international humanitarian law emerged, as the law of war began to afford more importance to the rights of the individual.