Today, 07:52 AM
It is a simple notion based on the observable fact that more powerful entities prevail over less, generally speaking.
That which constitutes "more powerful" is not a standardized list. Circumstance plays a role, sometimes significant. Size has nothing perforce to do with it, either. The lousy little island of Great Britain and the even smaller splotch known as the Netherlands are examples of more powerful entities that took names across the globe.
The key lies largely in organization, both the amount and the nature of which determines much of the relative power between organisms. Nations are, to all practical effect, organisms. They are larger than individuals, usually far more so. They tend to be more organized; certainly more broadly so such that other notions such as that of the division of labor come into prominence as well. That is where the real differentiation between smaller and larger groups begins to make itself felt.
Larger groups with a division of labor optimized for centralized control holds the potential for far greater power than those of more broadly distributed and/or smaller organisms. But there is a catch: there must exist the drive "at the top" to acquire, cultivate, maintain, and exercise this control. Similarly, there must be the will by the broad population to submit. But when both conditions are met in optimal/optimum form, the result is precisely small splotches of land moving outward in conquest of areas and populations hundreds or even thousands of times their own physical size.
Once again we can readily see that human weakness and other failings are a key element in enabling such organisms to enjoy such success. But it's not all bad, in a sense. Hearkening back to my notion of the least common denominator: once one nation arises and organizes along the lines here mentioned, the rest of humanity is presented with the choice to follow suit or be consumed. That is the way it happened on this world, at any rate. Those people who plunged themselves into the muck and filth of Empire survived. Those who did not were consumed, at times unto extinction.