The fate of empires is very often sealed by the interaction of war and debt. That was true of the British Empire, whose finances deteriorated from the First World War onwards, and of the Soviet Union. Defeat in Afghanistan and the economic burden of trying to respond to Reagan's technically flawed but politically extremely effective Star Wars programme were vital factors in triggering the Soviet collapse. Despite its insistent exceptionalism, America is no different. The Iraq War and the credit bubble have fatally undermined America's economic primacy. The US will continue to be the world's largest economy for a while longer, but it will be the new rising powers that, once the crisis is over, buy up what remains intact in the wreckage of America's financial system.
I can't think of a single word to add.