The unipolar era, a time of un­precedented American dominion, is over. It lasted some two decades, little more than a moment in historical terms.

Why did it end? One explanation is history. States get better at generating and piecing together the human, financial and technological resources that lead to productivity and prosperity. The same holds for companies and other organisations. The rise of new powers cannot be stopped. The result is an ever larger number of actors able to exert influence regionally or globally. It is not that the US has grown weaker, but that many other entities have grown much stronger.