Should Law govern disputes between Nations?

If there is to be an International Law, there has to be an International 'domain'. Law governs relations between individuals within a Democratic Nation. That law must be Equitable and Just. By that I mean, the individual's Freedom and Equality should never be compromised. However, on the International 'Domain', the issue is different. The International Domain must be constituted as a 'Collection of separate organizations' of different Nations, and each Nation is different, and that difference must be respected. The question immediately arises, are all the Nations in the world, members of the United Nations? Has 'someone' been left out; were they recognized as a 'constituted' Nation and refused entry; if refused entry, on what was the refusal based; is 'membership' still a viable option? If International Law applies to every Nation in the World, then every Nation in the world must become a member. If a Nation is left out, how can it be subject to International law? It can't. But, there are other problems within the concept of International Law. Each nation must be respected with respect to its form of government. Why? Because each Nation has a responsibility to its People and they are governed according to their own National policies and Laws. So how can a United Nations interfere with a Nation, who is not a member, and legally compel it to adhere to International standards. Well, it can't. That raises an interesting question, can 'democracy', within any one Nation, be compelled? Should democracy be a condition for joining an International organization of Nations? Is there such a thing as crossing a 'red line', where one Nation should not be tolerant of the conduct of another Nation's conduct toward its own People? If the sanctity (freedom and Equality)of the human condition exists within a National sphere, it should exists within an International sphere.