Alliances are as old as civilisation. Older, actually: almost every hunter-gatherer band that anthropologists have studied, from the New Guinea highlanders to the Yanomamo in the Amazon, made alliances with other groups to try to protect themselves.
But they often also ended up fighting people they had no quarrel with. "The enemy of my enemy is my friend" is the usual logic that alliances are built on, but people tend to overlook the fact that alliances also mean that "the enemy of my ally is my enemy too".
