This article is an exploration of the effectiveness of defence cooperation as a means of preventive diplomacy. The paper begins by suggesting that both defence cooperation and preventive diplomacy are concepts rooted in cooperative security. For the purposes of this paper, cooperative security is understood as an overarching concept that comprises alliances, collective security and preventive action. The fundamental claim of the article is that defence cooperation has more to offer than its own immediate benefits. The paper discusses how that value can be exploited towards a larger project of preventive diplomacy. As an illustration of these possibilities, the Indian experience of defence cooperation in the context of South Asia is also discussed.