Realist and rationalist approaches to balance of power politics underplay the degree to which balances are socially constructed. We develop a constructivist approach that accounts for the elusive and contentious nature of the balances that states seek to balance. The approach foregrounds contests over balance interpretations between states that shape whose conceptions and assessments underpin the making and remaking of balances. We argue that shifts in balancing practices are crucial to the dynamics of these contests. To substantiate this argument, we empirically study the contests over the conventional and theatre nuclear balances between East and West in the last two decades of the Cold War. The case study shows that the conventional and theatre nuclear arms control negotiations – that is, the partial shift from adversarial to associational balancing – initially fuelled and amplified both contests. At the same time, the arms control negotiations eventually resolved the contests through the development of shared understandings of the two balances, thus ending the adversarial balancing between East and West. Overall, we contribute to theory development on balance of power politics by highlighting the importance of contests over balance interpretations and by providing insights into the politics and dynamics that shape these contests.