The breakdown of the Soviet Union surprised most scholars of international relations, comparative politics, and Soviet politics. Existing explanations attribute the breakdown of the Soviet Union to the reformist leadership of Gorbachev, and/or to systemic factors. These explanations do not focus on the key contribution of the war in Afghanistan. This is surprising since many scholars view wars as key causal factors in empire breakdown and regime change. We argue that the war in Afghanistan was a key factor, though not the only cause, in the breakdown of the Soviet Union. The war impacted Soviet politics in four reinforcing ways: (1) Perception effects: it changed the perceptions of leaders about the efficacy of using the military to hold the empire together and to intervene in foreign countries; (2) Military effects: it discredited the Red Army, created cleavage between the party and the military, and demonstrated that the Red Army was not invincible, which emboldened the non Russian republics to push for independence; (3) Legitimacy effects: it provided non-Russians with a common cause to demand independence since they viewed this war as a Russian war fought by non Russians against Afghans; and (4) Participation effects: it created new forms of political participation, started to transform the press/media before glasnost, initiated the first shots of glasnost, and created a significant mass of war veterans (Afghansti) who formed new civil organizations weakening the political hegemony of the communist party.