Cooperation and discord in world politics are explained to a considerable extent by the three factors discussed in the Introduction: mutuality of interest, the shadow of the future, and the number of players. Yet the context of interaction, perceptions, and strategies is also important. Issues are linked to one another through multilevel games, which may be compatible or incompatible. Whether reciprocity constitutes an effective strategy depends both on linkages among issues and on the institutions within which negotiations take place. Perceptions are always significant and often decisive. Decision makers often actively seek to change the contexts within which they act by linking issues, trying to alter others? perceptions, establishing institutions, and promoting new norms. This finding suggests the importance of linking the upward-looking theory of strategy with the downward-looking theory of regimes.