What will be the role of the US in the 21st century? This is a question I rashly agreed to address last week at the Carnegie Council in New York. In analysing it, I considered a closely related issue that also exercises Americans: is the future role of the US in its own hands? The answer is: yes, but only up to a point. The US can control what it does. But it cannot control what others do.

The historic dominance of the US is the fruit of its exceptional assets. It is a continental power bounded by oceans to the east and west, and unthreatening neighbours to the north and south. It has huge, albeit dwindling, natural resources. It has had the world’s largest economy and the highest output per head since the late 19th century. The market-driven US economy has also been the world’s most innovative since at least the same era.

The US is home to the world’s most influential financial markets, albeit ones that triggered the Great Depression and Great Recession of recent years. It has been the issuer of the world’s main reserve currency since the first world war. It has offered one of the largest import markets, surpassed only by external imports of the EU.