Even the Roman empire at its zenith did not have the international influence of 21st-century America. Yet foreign policy analysts are increasingly asking why the US, with all its hegemonic power, seems unable to get its way in suppressing the Iraqi insurgency, ­stabilising Afghanistan, getting Iran and North Korea to abandon their nuclear weapons programmes and resolving the Israeli-Palestinian conflict. In other words, the magnitude of American power and Washington's inability to use that power to get its way in international politics presents aparadox.

Make no mistake: the US is indeed a global hegemon because of its overwhelming economic and military dominance. The sheer magnitude of America's military superiority dissuades even its closest would-be rivals from openly striving to compete as an equal.