The prohibition on the use of expanding bullets was one of the first prohibitions to arise in contemporary international humanitarian law. This prohibition is not a mere historical curiosity and retains considerable importance due to the breadth of legal instruments that it gave birth to. The spirit that drove the negotiations of the 1899 First Hague Peace Conference – growing concern about the horrors created by new military technologies and their increased lethality – has continued to inspire those who wish to advance the agenda of humanitarian arms control and disarmament today. This article contributes to the literature examining weapons prohibitions by engaging with the historical diplomatic and medical literature and debates that influenced the prohibition of expanding bullets in armed conflicts. It examines the outcry against the use of these bullets, contrasts it with the interests of the States that chose to use or not use them, and traces their influence on the 1899 Hague Conference and contemporary legal instruments. In doing so, the article highlights the dual influence of ethics and military interests in the prohibition of the use of expanding bullets in armed conflicts and argues that the creation of norms is more multifaceted than typically accounted for.

Riot control agents and expanding bullets are the only two kinds of weapon and ammunition that are used for law enforcement purposes but are explicitly prohibited in the conduct of hostilities. This article justifies this difference in treatment with two arguments. First, riot control agents and expanding bullets have different effects on the human body depending on their specific types and the circumstances in which they are deployed. Second, the issues raised by their use differ according to whether they are employed for law enforcement purposes or in the conduct of hostilities.