Americans are fond of “leadership” — both the word and the thing — but others can be forgiven for taking a dimmer view of it. “Leadership” means having informal control over others without submitting to formal vetting by them. As Johns Hopkins University professor Michael Mandelbaum argues in The Frugal Superpower, the leadership the US has exercised since the second world war, and particularly since the cold war, has made it “the world's de facto government”.

The US keeps order in ways that you see on television every night and in ways that you don't. It protects economic resources — particularly oil and the shipping lanes through which it moves. The US provides the global economy with a regulatory framework and a reserve currency. No one voted for this world “government”, at least not formally, and its legitimacy has been sharply questioned since the invasion of Iraq in 2003. Nonetheless, Mr Mandelbaum argues: “American power confers benefits on most inhabitants of the planet, even on many who dislike it and some who actively oppose it.” They would suffer were US power to diminish.

That is exactly what is happening now, Mr Mandelbaum writes. The US welfare state is overburdened, now the baby boom generation is retiring. Money to pay the pension cheques will be found by cutting defence and diplomatic budgets. The financial crisis that began with the collapse of Lehman Brothers straitened American resources even further. It shook US voters' confidence in governing elites (to the continuing surprise of the elites themselves), and led to massive new deficits. A recent Pew survey found that 49 per cent of Americans think the US ought to “mind its own business internationally” — a higher score than at any time since the question was first asked in 1964.