Between roughly 1950 and 1980, a group of theorists clustered around the British Committee on the Theory of International Politics — Hedley Bull, Martin Wight, Adam Watson, and others, several at the London School of Economics — developed an intermediate position between realism and liberal institutionalism. They called it the English School, and its central concept was the international society — a system of states that share enough common interests, common rules, and common institutions to be more than a Hobbesian jungle, but not so unified as to be a world state. The school's distinctive temperament was historical rather than formal-modeling: it read the European states-system as a centuries-long civilization that had slowly taught its members how to coexist. International society sits between the jungle and the world-state — a halfway civilization.
The English School's analytical move was to take international institutions seriously without idealizing them. Bull's The Anarchical Society (1977) argues that even in the absence of a world government, states maintain order through five primary institutions: the balance of power, international law, diplomacy, war as a regulating mechanism, and the great powers. These institutions function imperfectly, are violated frequently, and have been built over centuries — from the residency embassies of Renaissance Italy to the Congress system after 1815 — but they exist, and they make international life qualitatively different from sheer anarchy. The school distinguishes between pluralist international society (states agree only on minimal rules of coexistence — sovereignty, non-intervention, diplomacy) and solidarist international society (states agree on substantive values — human rights, humanitarian intervention, common rules of justice). Most of the post-1945 system has been pluralist with solidarist aspirations; the high-water mark came in the 1990s, with the ad-hoc tribunals for Rwanda and Yugoslavia and the 1998 Rome Statute. The long argument over humanitarian intervention, responsibility to protect, and international criminal justice is essentially a debate about how much solidarism the system can sustain before it fractures the consensus that holds it together.
The current strain on the rules-based international order is, in English School terms, a contraction of solidarist commitments and a partial return to pluralist minimums. The great-power return — Russian revisionism, Chinese assertion, American transactionalism — is being absorbed by the underlying institutions of pluralist coexistence (sovereignty, diplomacy, balance of power, the great-power club itself), even as the more ambitious machinery seizes up. Whether the solidarist project of the 1990s — international criminal courts, R2P, universal human rights — survives this contraction, or shrinks into a regional achievement of the liberal democracies alone, is one of the genuinely open theoretical and practical questions.