http://gametheory101.com/courses/international-relations-101/ Cooperation cannot occur if states know when their prisoner's dilemma interaction will end. What if the end of the interaction is uncertain? Cooperation is possible if the states adopt a policy of tough love---a.k.a. a grim trigger strategy. The states begin by cooperating. Then, as long as everyone has cooperated in the past, they continue cooperating. Otherwise, they defect. The Evolution of Cooperation by Robert Axelrod is a classic book on this subject: http://www.amazon.com/gp/product/0465005640/ref=as_li_ss_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=0465005640&linkCode=as2&tag=gamthe101-20