A central thread in the geopolitical story of the past few years has been American power and American vulnerability. In the global maelstrom since September 11 2001 the rest of the world has at once chafed at US might and quietly celebrated its pregnability. The paradox, visible again in the grisly aftermath of Hurricane Katrina, explains much of the confusion in international affairs.

There are many ways of measuring US primacy. Most obviously, America spends more on its military than every other nation combined. Look around the world and more often than not the US is the dominant regional power, an indispensable presence in balancing local rivalries. American economic power, meanwhile, is reflected not only in an unrivalled wealth but by seemingly unassailable leads in science, technology and innovation. To the political and the economic, add an extraordinary cultural reach. Like it or not, young people from Beijing to Brussels and Reykjavik to Rio de Janeiro look to the US for their lifestyles.