[xyzzyz]

Note that I said "[t]he nation of Spain", not the "rulers" of Spain. It's exactly the same as in US: in both cases, the nations codified the rules through the constitution they made the law. If anything, it is more of the case with Americans who are given down rules from above: the American constitution was created and voted into the law by the representatives of the people, while in Spain, the people themselves voted the current constitution into the law through public referendum.

[vidarh]

The point is that in the case of most modern nations, they did not exist as separate nations until they asserted that they were, often by waging war on those who insisted they were not.

These nations bootstrap their nationhood, and are first legitimate after the fact.

One would hope that we will extend civilization to the point where people who want independence don't need to kill to prove they're serious.