The purpose of this article is to present and establish the significance of the Nuclear Arsenal Game (NAG). The NAG investigates behaviour within dyads composed of states that experience an international crisis. It assumes that nuclear and quasi–nuclear states act according to the size and potential of their own nuclear force structure and to that of their opponent. This article argues that the size and potential damage an arsenal poses, set in relation to an enemy state's second–strike capability, determines actor preferences within a crisis situation, preferences which include the option of launching a first strike and risking retaliation in kind. The primary purpose here is to propose a nuclear index for use in empirical studies and offer an example of a game–theoretic model of crisis interaction based on the Theory of Moves (TOM) that considers (a) all levels of nuclear capability; and (b) Classical (or Rational) Deterrence Theory (CDT) to be effective in identifying and explaining actor preferences and predicting behaviour that could include intentional nuclear use.