This article proposes a hybrid legal framework combining jus ad bellum and jus in bello to govern the attribution of State responsibility for reparations at the end of a war of aggression. To this end, the article considers former international mass claims processes and proposes a complementary approach that, on the one hand, acknowledges the role of the aggressor State in waging the war, and on the other, takes a cautionary approach to prevent a disproportionate burden of compensation being imposed on the aggressor State as a form of collective punishment. The consequences of respective violations of the prohibition of the use of force and the law of war are blurred in a war of aggression, resulting in complexities around liability for aggressor States. In response, this article concludes with a nuanced proposal to calculate compensation based on (1) the aggressor party's capacity to comply with jus in bello; (2) the extent of damage caused by the war of aggression, factoring in jus ad bellum considerations if a party is found to be intentionally maximizing destruction; and (3) the incorporation of tort law principles for equitable attribution of responsibility.