This article aims to offer a systematic legal assessment of Kenya's invocation of the right of self-defence in its 2011 intervention in Somalia, Operation Linda Nchi. First, the article will establish the existing jus ad bellum framework, with particular emphasis on the permissibility of actions in self-defence against attacks carried out by non-state actors. The article will then evaluate the 2011 Kenyan military operation in light of the three core principles governing the lawful exercise of the right of self-defence-that is to say necessity, proportionality and immediacy-criteria found under customary international law. Throughout this assessment, the intervention will be compared with similar military operations carried out in response to attacks perpetrated by non-state actors. By contrasting the 2011 intervention with comparable military operations, the article aims to draw conclusions as to its legality under international law. Finally, the article will make suggestions as to the impact of the invasion upon the jus ad bellum, particularly as regards the right to use force in self-defence against non-state actors.