One hears a false version of these principles from the UN and its apologists in the form of: "Every nation has the right to defend itself." No. Every democratic nation has a right to defend itself and be free of unwarranted outside force on the basis of its own legitimacy.
Fine. How does one define the proper style of democracy? Is it American-style constitutionalism, British Parliamentarism, or something different? Should it be the sort of radical democracy that Anarchists tend to advocate, or a minarchist system that you tend to see argued for online? Are there term limits? Does the head of state have to be elected? Can a state religion exist, and if so which one? Can private property exist, and if it can should it be protected as a natural right outside the purview of the government, or just by the decisions of the electorate? (Rousseau, Hobbes, and Locke would have very different answers to this point.) And what about the tyranny of the majority? How do we account for it? And what about the socialists and communists who argue that the state is just a tool by one class to exploit another, and believe that democracy is pointless until the classless society comes to be?
More to the point, who gets to decide this, and why? And if you disagree with them, do they get to blow your head off, or do you get to blow their heads off? Or does it come down to who has the bigger guns? Not a recipe for a safe international community, especially if people take these questions deadly seriously. And, yes, they do.
That's why the quotation above is no falsehood- because there is no universal standard on what makes a good government, and nobody can stand from on high and decide "this is real democracy, and that is not". That's why the UN doesn't make the mistake that the League of Nations did (which people today forget) of only letting in governments that it likes. National sovereignty exists for a damned good reason, and as much as we'd like to reach out and change other governments into the kind we'd like, we can't do so and not leave ourselves open to being "adjusted" by others as well. Then, well, it just comes down to who shoots fastest.