Introduction

This article considers the relationship between geopolitical change and the evolving international rules on military force. Its focus is the impact of the United States’ rise to hegemonic status on the rules governing recourse to force (the jus ad bellum) and the conduct of hostilities (the jus in bello, otherwise known as ‘international humanitarian law’). For reasons of space and clarity of analysis, the article does not focus on the different, more traditional IR questions of whether and why the behaviour of the United States might be constrained by these rules.

Two specific sets of rules are examined: the right of self-defence and the rules governing the treatment of detainees. The article concludes that geopolitical change frequently leads to normative change, though on some issues – such as pre-emptive self-defence – even a hegemonic state cannot change international law on its own. The challenge facing the international community is to maintain rules on military action that are reasonable, effective and widely accepted – including by the most powerful state, at least most of the time.

Self-defence

The right of self-defence in contemporary international law dates back to 1837, when the British were crushing a rebellion in Upper Canada (now Ontario). The United States, while unwilling to antagonise a superpower by supporting the rebels directly, did not prevent a private militia from being formed in up-state New York. The ‘volunteers’ used a steamboat, the Caroline, to transport arms and men to the rebel headquarters on Navy Island, on the Canadian side of the Niagara River.