Is America the new world empire?Presidents from Lincoln to Bush may have denied it but, as Niall Ferguson's brilliant and provocative book shows, the US is the greatest military and economic colossus of all time. What's more, it always has been an empire, with its founding fathers battling westwards for territory and their successors spreading freedom across the world - at gunpoint if necessary.Yet is the US really equipped to play Atlas, bearing the weight of the world on its shoulders? America, Ferguson reveals, is now an empire running on empty, backing away from the crucial imperial commitments of time, money and manpower - and resting on perilous financial foundations. When the New Rome falls, its collapse may come from within.