They never learn
Europeans have been in a state of almost constant warfare since the Pax Romana, about two thousand years ago. The last two, the 20th century's World Wars, dragged the entire planet along with them.
If it doesn't quit arguing about the small stuff and unify politically and economically Western Civilization will destroy itself. The nation-state is a fundamentally unstable form of government, and you can't trust old white guys with gunpowder, and certainly not with nukes.