The purpose of this thesis is to assess Pakistan’s relationship with Afghanistan before and after the Soviet invasion of Afghanistan in 1979. I argue that the nature of the relationship was transformed by the region becoming the centre of Cold War conflict, and show how Pakistan’s role affected the development of the mujahidin insurgency against Soviet occupation. My inquiry begins by assessing the historical determinants of the relationship, arising from the colonial legacy and local interpretations of the contested spheres of legitimacy proffered by state, tribe and Islam. I then map the trajectory of the relationship from Pakistan’s independence in 1947, showing how the retreat of great power rivalry following British withdrawal from the subcontinent allowed for the framing of the relationship in primarily bilateral terms. The ascendance of bilateral factors opened greater possibilities for accommodation than had previously existed, though the relationship struggled to free itself of inherited colonial disputes, represented by the Pashtunistan issue. The most promising attempt to resolve the dispute came to an end with the communist coup and subsequent Soviet invasion, which subsumed bilateral concerns under the framework of Cold War confrontation. Viewing the invasion as a major threat, Pakistan pursued negotiations for Soviet withdrawal, aligned itself with the US and gave clandestine support to the mujahidin insurgency. External support enhanced mujahidin military viability while exacerbating weaknesses in political organization and ideology. Soviet withdrawal in 1989 left an unresolved conflict. Faced with state collapse and turmoil across the border, heightened security concerns following loss of US support, and intensified links among non-state actors on both sides of the frontier, the Pakistan government drew on its recently gained experience of working through non-state actors to attempt to maintain its influence in Afghanistan. There would be no return to the relatively stable state-state ties prevailing before 1979.