Victory in the Cold War made it the hegemon in a unipolar world.

Few argued when the 20th Century was dubbed the "American Century", a term first coined in the early 1940s when the country was still overcoming its isolationist instincts.

Even the New Year's fireworks, which illuminated the obelisk of the Washington Monument in a way that made it resemble a giant number one, projected the country's supremacy as the world's sole superpower.

Over the past 15 years, America's fortunes have changed with dizzying speed.

First came the tremors: the dot-com bust and a disputed presidential election in 2000. Then came the massive convulsions: the destruction of the Twin Towers in 2001 and the collapse of Lehman Brothers in 2008.

Long wars in Afghanistan and Iraq have exacted an enormous blood price - the lives of 6,852 American military personnel - not to mention immense financial expense, estimated to be as high as $6 trillion (£3.9tn).