The three-decades old call for an inter-disciplinary rapprochement between IR Theory and Historical Sociology, starting in the context of the postpositivist debate in the 1980s, has generated a proliferating repertory of contending paradigms within the field of IR, including Neo-Weberian, PostStructuralist, and Constructivist approaches. Within the Marxist literature, this project comprises an equally rich and diverse set of theoretical traditions, including World-Systems Theory, Neo-Gramscian IR/IPE, the Amsterdam School, Political Marxism, Neo-Leninism, and Postcolonial Theory. More recently, a “third wave” of approaches has been announced from within the field of IR, suggesting to move the dialogue from inter-disciplinarity towards an integrated superdiscipline of International Historical Sociology (IHS). This proposition has been most persistently advanced by advocates of the theory of Uneven and Combined Development (UCD), claiming to constitute a universal, unitary and sociological theory of IR. This article charts the intellectual trajectory of this ongoing IR/HS dialogue. It moves from a critique of Neo-Weberianism to a critique of UCD against the background of the original promise of the turn in IR to Historical Sociology: the supersession of the prevailing rationalism, structuralism, and positivism in extant mainstream IR approaches through the mobilization of alternative and non-positivistic traditions in the social sciences. This critique will be performed by setting UCD in dialogue with Political Marxism. By anchoring both approaches at opposite ends on the spectrum of Marxist conceptions of social science – respectively the scientistic and the historicist - the argument is that UCD reneges on the promise of Historical Sociology for IR by re-aligning, first by default and now by design, with the meta-theoretical premises of Neo-Realism. This is most visibly expressed in the articulation of a deductive-nomological covering law, leading towards acute conceptual and ontological anachronisms, premised on the radical de-historicisation of the fields of ontology, conceptuality and disciplinarity. This includes the semantic neutering and hyper-abstract rearticulation of the very category, which in IR’s self-perception lends legitimacy to its claim of disciplinary distinctiveness: the international. The article concludes by suggesting that an understanding of Marxism as a historicist social science subverts all calls for the construction of grand theories and, a fortiori, a unitary super-discipline of IHS, premised on a set of universal, space-time indifferent, and abstract categories that hold across the spectrum of world history. In contrast, recovering the historicist credentials of Marxism demands a constant temporalisation and specification of the fields of ontology, agency, conceptuality and disciplinarity. The objective is to lay the foundations for a historicist social science of geopolitics