In parallel to the drawing up of an illegality typology, the team undertakes an extensive study of state practice related to armed conflicts since 1945. The analysis shall reveal what states have been contesting in actual conflicts and with which legal arguments. This in turn, shall inform the broader typology of illegality, in particular the prevalence of the respective types of illegality in the practice of states. The analysis will also reveal when and under which circumstances challenges to the law turn out to be 'successful', i.e. result in a change of existing legal principles.