If a week is a long time in politics, a decade is starting to look like an age in geopolitics. Comparing the America that began the 21st century with the America of today is to witness a country that has in some ways quite radically altered its view of itself and its relationship to the world.

In short, the metallic rust of decline has crept into the American soul. “You could argue that the first decade of the 21st century was the last decade of the American century,” says David Rothkopf, a former Clinton administration official and student of US foreign policy. “We are now entering the multipolar century.”

That change in perception is not readily backed up by statistics. With a rough 22 per cent share of global income, the US weighting in the global economy has barely shifted since 1975 when it hit its post-war nadir. Nor is it expressed in terms of declining US military power – quite the opposite: the Pentagon's budget remains larger than the next seven powers combined.