How did Fred Halliday recast International Relations (IR) theory as international historical sociology? This article explores Halliday's intellectual trajectory across this terrain and suggests that the notion of ‘capitalist modernity’, derived from an amalgamation of neo-Marxian and neo-Weberian historical sociology, functioned as the strategic master-category, which anchored his thought on International Relations throughout his work. This category was successively reconceived and complemented to generate four, partly contradictory, analytical frameworks at a lower level of abstraction: ‘global conjunctural analysis’; a neo-Weberian ‘sociology of the inter-state system’; ‘international society as homogeneity’ and ‘uneven and combined devel...