What is happening to the ‘rules-based international order’ despairingly invoked by bewildered European leaders? The broad answer is that we are living through the retreat of American hegemony, masked by bluster and marked by contradictions. The retreat has two aspects, economic and geopolitical. Economists talk about Trump’s tariffs breaking up the free-trade order; geopoliticians about the Trump Corollary breaking up the Nato system. These are part of a single, reasonably coherent story. But the retreat is not as straightforward as it sounds. How does the bombing of Iran fit into it?

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What do people mean when they talk about a global ‘rules-based order’? The starting point must be the UN Charter, signed in San Francisco in 1945, and in theory binding on all the UN’s 193 member states. The Charter enjoins sovereign legal equality (Article 2.1), peaceful settlement of disputes (Article 2.3), and prohibition of the threat or use of force (Article 2.4), unless for self-defence (Article 51) or when authorised by the Security Council (Chapter VII). Security on land, sea and in the air is the source from which flows all the other treaties and conventions which make up our ‘rules-based’ world.

But all international agreements, however solemn, share one defect: there is no world government to enforce them. So how has our ‘rules-based order’ worked in practice?

There are two answers. Charters, treaties, and conventions do have some binding force on those who sign up to them, just as the rule of law within nations does not wholly depend on the exertions of police forces. Nations make treaty commitments which it is in their interest to honour, since they make their relations more stable and predictable.

But, as I have argued previously, the international rule of law depends crucially on the existence of a leading power, able and willing to underwrite the rules and enforce them as required. The United States took over this hegemonic role from Great Britain after 1945; the ‘rules-based order’, whose passing is so much lamented, was in essence an Anglo-American creation. In the Cold War era, it was never truly global, rather a superpower duopoly. But for a decade or so after the collapse of Communism in 1990, the global role of the United States was unchallenged. We now refer to this short time as the ‘unipolar moment’.

There is a further fact which deserves emphasis: while the rule of law was enjoined on all nations, the hegemon retained discretion to break the law when it suited it, since there was no external constraint on its actions. It alone was truly sovereign in a world of supposedly equal sovereigns, and therefore in practice unaccountable.

But no single power can sustain such a role indefinitely. Commitments outstrip resources, the hegemon sloughs off its responsibilities, and the ‘world order’ which it guarantees falls apart. We are told that the hegemonic torch passed seamlessly from Britain to the United States. We forget the two world wars, which fatally weakened British........

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