The International Criminal Court (icc) will gain the capacity to prosecute the crime of aggression in 2017. The Amendments to the Rome Statute are the product of a political compromise and have a complex legal structure with a high definitional threshold for an act of aggression alongside a bespoke jurisdictional arrangement. This legal structure is likely to mean that very few acts of aggression are considered crimes. Even when acts of aggression pass the threshold set out in the amendments, it is highly likely that any such prosecution would not succeed. This article argues that this is likely to significantly impact the legitimacy of the icc as an organization. To understand this, it is necessary to look at the different meanings of legitimacy before examining how the way in which the law is configured could undermine the political legitimacy of the organization as a whole.