Collective Security
An institutional design, embodied by the League of Nations and the United Nations, in which a near-universal membership commits to organize a combined response against aggression from any state, including a member. Struggles specifically when a great power is the aggressor, since organizing a coalition strong enough to override one of the system's own most powerful members is far harder than organizing one against a minor power — a lesson the League learned directly in Manchuria (1931) and Ethiopia (1935), and the reason the UN Security Council's veto is a deliberate design response to that failure, not an oversight.