The causes of World War I remain a topic of enormous intellectual interest. Yet, despite the immensity of the literature, historiographical and IR debates remain mired within unhelpful methodological dichotomies revolving around whether a ‘primacy of foreign policy’ versus ‘primacy of domestic politics’ or systemic versus unit-level approach best account for the war’s origins. Given that this historiography is the most prolific body of literature for any war within the modern age, it reveals a much deeper problem with the social sciences: how to coherently integrate ‘external’ and ‘internal’ relations into a synthesized theory of inter-state conflict and war. Drawing on and contributing to the theory of uneven and combined development, this article challenges standard interpretations of the war by distinctively uniting geopolitical and sociological modes of explanation into a single framework. In doing so, the article highlights how the necessarily variegated character of interactive socio-historical development explains the inter-state rivalries leading to war. Contextualizing the sources of conflict within the broad developmental tendencies of the Long 19th Century (1789–1914) and their particular articulation during the immediate pre-war juncture, the article seeks to provide significant contributions to recent debates in IR and historical sociology, as well as those concerning the relationship between history and IR theory.