You have to understand that mainstream Anglo-American IR was, until very recently, built on the assumption that theorizing departs from the existence of the interstate system as a natural given, rather than something that requires
explanation in the first place. It posits the political as an autonomous sphere in which states are generically endowed with a unitary rationality and ascribed certain attributes, foremost survival, security, and hence power-maximization.
Once these axioms are in place, you can then establish by means of a series of logical deductions how rational state action in a condition of international anarchy leads to certain likely outcomes, including power-balancing, leading to some kind of self-equilibrating systemic logic. This is a nice little exercise in abstract logic, actually modeled, by Kenneth Waltz, in analogy to the workings of the anarchy of competitive markets self-regulated by the invisible hand. It is also said to be grounded in ancient wisdoms – si vis pacem para bellum – but bears hardly any relation to reality. So the works of Hans Morgenthau and later of Kenneth Waltz are really premised on drawing an analytical Rubicon between the state and systemic interstate relations and anything that goes on within societies within these states. So the domestic and the social are excised from the remit of what could count as possible influences on statecraft and foreign policy – party politics, business and sector interests, social crises and so on.
This is of course an incredibly narrow, impoverished and ideological way to think about international relations as a social science..."
Rethinking International Relations