> I was about to state that US GDP as a percentage of the world has been
> declining, but apparently it hasn't
Sure it has. At its peak of global dominance, the US generated half of global GDP in 1945. By 1999, this had fallen to 30%, and by 2009, to 25% (World Bank data, GDP at market exchange rates). Most of this decline isn't absolute but relative -- the much-delayed spread of industrialization, literacy and urbanization in the postcolonial nations.
In general, I'd say US policy elites are depressingly clueless about the multipolar world. The majority still think in the terms of Cold War agitprop: allies 100% good, enemies 100% bad. In reality, big, independent countries like the BRICs are not only getting rich, they're also developing thriving democracies (or in the case of China, undergoing internal democratization), and now have their own versions of Nye's "soft power", e.g. huge media audiences and sophisticated media cultures to tell their stories.
-- DRR