Specifically a simple act of revenge in Sarajevo began World War I – which even now no one can really explain other than the revenge concerning an assassination. Then came a decade of Laissez-faire capitalism run amok which caused a decade of world depression, resulting in turn with World War II which was also an act of revenge. Then came 45 years of most of the money in the world going to wars and military build up of the Cold War, and now we have 25 years of religious wars. Also driven by revenge.
As I recall someone came by a long time ago to address that revenge thing, they nailed him to a tree.