Abstract: “The Enemy of My Enemy is My Friend” – so it was said in ancient times, and so it is said, often, today. But is it rational to think and act that way? The meaning of the statement is unclear. So is its justification, and so its scope of application. In deciding whether to treat the enemy of one’s enemy as a friend, there is an analysis to undertake, involving the costs and benefits of doing so. To assess those costs and benefits, one needs to know what, exactly, it means to treat someone as a friend. There might also be deontological reasons not to treat some enemies of one’s enemies as friends. The statement makes clearest sense in the context of war: If someone is willing to fight on one’s side, he is the enemy of one’s enemy, and might be essential to one’s very survival. If so, he might be treated, at least for the relevant time, as if he is one’s friend (with mutual forbearance, genial relationships, certain norms of reciprocity, and perhaps pooling of resources). “The Enemy of My Enemy Is My Friend” might be a rallying cry during some kind of battle. Note, however, that the enemy of my enemy need not be my literal friend; this might simply be an alliance, and a temporary one, of selfinterest. In contexts that do not involve war or (literal) battle, one might accept the enemy of one’s enemy as a friend, in some sense, in order to widen the tent and strengthen one’s capacity (perhaps in politics, perhaps in intellectual life, perhaps in a neighborhood). Friendship, in this context, might refer to a degree of collaboration and reciprocity, and again some pooling of resources. But there are pervasive problems. Among other things, the enemy of one’s enemy might really be an enemy, in some intelligible sense, and if one enters into an alliance, one might dirty one’s hands or put at risk one’s deepest values, and potentially one’s survival, at some later date.