This, from NBS, is perfect.
Asked (by Garrett Gray): “What reason is there to think there’s an irreducible anarchy between sovereigns?” he responds —
Suppose there is no anarchy between sovereigns. This means there is a law governing sovereigns. Which means there is a sovereign over the sovereigns. Which means that the sovereigns weren’t sovereign. Which is a contradiction. Therefore there IS anarchy between sovereigns.
This insight is already the solid foundation of IRT, but it’s surprising how few seem to clearly get it.