On the History of North America Largely Being a Tale Where, for CENTURIES, Clusters of Powerful People (Both In Terms of Numbers and Technological Superiority) Displaced Clusters of Less Powerful Ones, Thereby Expanding Their Territory

And, of course, it also helps greatly if the people being displaced just lay down and let the shit happen (the opposite of what Harold Godwinson did circa 1066, in other words). Not that it's a common event, mind you, but, alas, it HAS happened from time to time.