"The American empire has passed its zenith."

This statement has a whole lot of issues associated with it. Anyone care to examine it?
At the risk of getting flamed I will take a shot.
This all depends on when you consider America to have been at its strongest. I would argue 1945 (over 100 aircraft carriers in the bay of Toyko, only country which can make nukes, controls much of worlds production) but many others would suggest right after the fall of communism. Until the fall of communism America had always had another superpower to compete with. Now America is the lone superpower. It is clear that for at least 20 years China will not be able to touch America in terms of military power.
In terms of economic power the situation America was in in 1945 will never be repeated. There is zero chance America will control that % of the worlds industrial production. Read the first few pages of Bill Brysons "The Life and Times of the Thunderbolt Kid (2006)" (Not the most academic source but the way he writes about American economic power is amazing).
Just because America has most probabaly passed its zeinth (only future historians will know) does not mean that all it lost. It can continue to be the strongest country in the world but other countries will catch up faster than America can advance (it is harder to lead, just look at Ger, the US, and Jap catching up to the UK in the indy rev).

All this being said the biggest threat I see to America is economic. They need to get their house in order.