Published by Princeton University Press, Princeton, N.J., 1998. At first blush, Zakaria says, the answer seems easy. Nations get rich, they amass large armies, attack other, usually weaker, nations, colonize them, and eventually challenge other powerful nations in the world, self-styling themselves as the protector of weaker nations.
Nations thus expand when they can.
Since all nations seek control over their territory, behavior of other states, and world economy, a significant rise in one's own material resources, classical realism theorizes, immediately affects its desire to influence world economy. This, in turn, has repercussions in the nation's desire to seek control over other states who, also powerful, also seek to affect the world economy. According to Classical Realism, it is this which prompts nations to conquer and colonize. Defensive realism is a bit more benign, Zakaria says, but, in the end, unconvincing. It expounds that nations expand because they are insecure.