Theory of Governance
A nation is a territory within which a body of law applies. A stable nation must delegate to its agents the power to maintain its territory and enforce its law. Nations that do not maintain their territory tend to be annexed by other nations; nations that do not enforce their law tend to be replaced from within. National instability is therefore usually a temporary condition, but periods of it can still cause harm to subjects of the nation.
"Territory" here means the area which the nation in fact controls. A government in exile does not have territory and is therefore not a nation until it returns or gains territory elsewhere. Areas where people can live are controlled by one nation or another almost without exception.
"Law," similarly, means the law which the nation in fact makes apply. A so-called law that the officials of a government do nothing to enforce is merely a piece of paper. Conversely, a policy that the nation does actual work to enact is effectively law regardless of its origin. A nation's law may be totally unrelated to its written legal code, or have very little overlap.
A "subject" of a nation is a person whom the nation is capable of acting upon. This includes residents, passers-through, and legal citizens living or traveling abroad. This also includes people in territories that the nation is seriously contesting, anyone whom the nation has imprisoned for any reason, and anyone who is liable to be extradited to the nation. Nations have certain natural responsibilities toward their subjects.
An "agent" of a nation is a person through whom the nation acts. As a rule of thumb, if someone could say that the government did this or that, but the material components of the action were completed by people acting as authorized by national law, those people are agents of the nation. Agents operate by the sufferance of the nation and an agent who breaks the law is likely to lose their license.
To a significant extent, these concepts as they are actually enacted do line up with national documents. However, changing the document does not necessarily change the reality. It's more common for documents to describe the workings of a nation than for the workings of the nation to exist as an expression of particular documents.