http://gametheory101.com/courses/international-relations-101/ In one-shot prisoner's dilemma interactions, states cannot cooperate because each has individual incentives to defect on the other. However, some interactions take place over the long term. Can states cooperate at the beginning under the threat of defection later on? Unfortunately, the answer is no. At the end of the interaction, the states must defect on one another. The shadow of the future casts a pall on the present, leading to defection throughout. This holds no matter how long the interaction is.