"The enemy of my enemy is my friend." This common adage, which seems to be adhered to in social interactions (e.g. high school cliques or work relationships) as well as in political alliances within countries and between countries, describes the ability of groups or people to work together when...

This paper presents a game-theoretic rationale for the beneficial effect of a common enemy on cooperation. In a defence game against a common natural threat, the value of the public good of defence is equal to the sum of the players’ defensive efforts. The game therefore takes the form of...