"In order to have the best intelligence in the world to know who the terrorists are and where they are and what they're plotting, you've got to have the best cooperation you've ever had in the world."

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Repetition acts as an enforcement mechanism: It makes cooperation achievable when it is not achievable in the one-shot game, even when one replaces strategic equilibrium as the criterion for achievability by the more stringent requirement of perfect equilibrium.

— Robert Aumann