The primary international political organization is the sovereign state (since the Peace of Westphalia - 1648). The 20th Century was a period of radical expansion of independant states, but as different as each of these entities may be, they share many characteristics in common: sovereignty, territory, population, armed forces, and the recognition of other states. Under international law the state is the only form of organization that has a legal right to use force.

It is important to note that a state is not necessarily identifiable as a nation. In short, the former is a legal entity and the latter is a perceived entity (ethnic, language, etc.). The unfortunate confusion that arises is when people attempt to define all states as nation-states.

I admit double checking my terms and wording with Games Nations Play, 6th Ed. 1987.