Accidental Adversaries in History

Human history is a long story often made longer…and less interesting…by being reduced to a litany of facts. (Fact: a piece of information which, albeit true, is meaningless.) The real story of history deals with “how” and “why.”  One very under-reported answer is some version of the system dynamics archetype “accidental adversaries.” It would be nice to know how much of human history could be understood by knowing how often conflicts were the result of accident, e.g., of friendship broken even when the two sides in fact had the same interests.

Can anyone think of an example?