This dissertation explores the determinants of state defense industrial capacity, advancing a theory to explain its variation between countries and across time. Defense industrial capacity is conceptualized as having two aspects: procurement capacity (the capacity of the state to acquire a military force structure tailored to its strategic requirements), and gatekeeper capacity (the capacity of the state to leverage defense industrial production to achieve international influence). Variation in both aspects of defense industrial capacity are argued to rest on two factors: the structural position of the state within both domestic and international defense industrial networks (which affects the balance of influence of weapons producers versus customers), and the breadth of military technological requirements (affecting the ease of emulation and hence the incentive to innovate). It is contended that changes in these factors drive shifts in state defense industrial capacity, via cycles in defense industrial demand linked to the evolution of the international strategic environment. The utility of this theoretical framework is demonstrated through application to four cases. The first two centre on Great Britain. Between 1888 and 1900, the strong structural position of the British state combined with widely distributed technological capabilities resulted in an effective procurement response to the Franco-Russian strategic challenge, while limiting the ability of Britain to exploit its global defense industrial position for influence. Conversely, the rising centrality of producers reduced the procurement autonomy of the British state between 1904 and 1913, encouraging a strategically counterproductive qualitative naval arms race with Germany. The subsequent two cases focus on the US experience from the late Cold War onward. Between 1979 and 1997 the US government harnessed its domestic defense industrial influence to lever- age more complex military technological requirements in the service of innovation, increasing strategic pressure on the Soviet Union and deepening US centrality within the global defense industrial network. However, from 2002 to 2010 the structural position of the US government was eroded, allowing private preferences for innovation to shape military procurement out of step with post-9/11 strategic requirements. The dissertation concludes with a consideration of the applicability of the theory to contemporary US-China strategic dynamics.