That, of course, is all history, but the Pax Americana that has taken shape since 1989 is just as vulnerable to historical change. In the 1910s, the rising power and wealth of Germany and America splintered the Pax Britannica; in the 2010s, east Asia will do the same to the Pax Americana.
The 21st century will see technological change on an astonishing scale. It may even transform what it means to be human. But in the short term – the next 20 years – the world will still be dominated by the doings of nation-states and the central issue will be the rise of the east.
By 2030, the world will be more complicated, divided between a broad American sphere of influence in Europe, the Middle East and south Asia, and a Chinese sphere in east Asia and Africa. Even within its own sphere, the US will face new challenges from former peripheries. The large, educated populations of Poland, Turkey, Brazil and their neighbours will come into their own and Russia will continue its revival.
Nevertheless, America will probably remain the world's major power. The critics who wrote off the US during the depression of the 1930s and the stagflation of the 1970s lived to see it bounce back to defeat the Nazis in the 1940s and the Soviets in the 1980s. America's financial problems will surely deepen through the 2010s, but the 2020s could bring another Roosevelt or Reagan.
A hundred years ago, as Britain's dominance eroded, rivals, particularly Germany, were emboldened to take ever-greater risks. The same will happen as American power erodes in the 2010s-20s. In 1999, for instance, Russia would never have dared attack a neighbour such as Georgia but in 2009 it took just such a chance.
The danger of such an adventure sparking a great power war in the 2010s is probably low; in the 2020s, it will be much greater.
The most serious threats will arise in the vortex of instability that stretches from Africa to central Asia. Most of the world's poorest people live here; climate change is wreaking its worst damage here; nuclear weapons are proliferating fastest here; and even in 2030, the great powers will still seek much of their energy here.
Here, the risk of Sino-American conflict will be greatest and here the balance of power will be decided.
Ian Morris, professor of history at Stanford University and the author of Why the West Rules – For Now (Profile Books)