Isabel V. Hull uses the German declarations of war in 1914 to examine three issues: 1) the role of customary international law (CIL) in statesmen’s decision to go to war (using Germany as an example); 2) the assumptions that state actors held a jus ad bellum; and, especially, 3) how they distinguished self-defence, prevention, pre-emption, and aggression. Hull uses not the claims of jurists, but the arguments and actions of civilian and military leaders, i.e. those who actually made the decisions for war. With this, she continues Anuschka Tischer’s and Hendrik Simon’s examination of the question whether there was a transformation of war discourses in (early) modernity that led to overcoming the need to justify war. The chapter confirms that, even as Germany began a ‘preventive war’, the European state consensus held that, on the continent, preventive war was illegal, pre-emptive war was severely restrained, and genuine self-defence – meaning both fending off armed attack against one’s territory, independence, or sovereignty, and defending the treaty-structure that guaranteed the inter-state order – was the only justification for war acceptable to the community of states.