Towards the end of next January, the US and Europe are going to wake up with a jolt. A new American president will be told that, for the first time in its history, the US is a nation entering relative decline. Europeans will discover simultaneously that the departure of George W. Bush has deprived them of an alibi.

Amid the stacks of briefing papers presented to John McCain or Barack Obama will be an assessment of the likely contours of the geopolitical landscape over the next 15 years. We can assume it will state the obvious: that if there was a unipolar moment after the end of the cold war, it passed as quickly as it emerged.

An important word in this analysis is “relative”. The US can expect to be the sole superpower for some time yet, if we mean by the term a state capable of deploying effective power almost anywhere in the world. Measured by economic weight, technological capability or military prowess, the US will remain the pre-eminent power. But the shift in its relative position vis a vis the rising nations of Asia, particularly China, will tighten the constraints on the exercise of its power.