Bullocks, England declined in the 20th century because:
* It became socialist and protectionist, which hindered growth and let the US outpace it
* It engaged in a very, very stupid and destructive war, the consequences of which brought about another stupid and destructive war
Two days before the NSS became public, the International Monetary Fund issued a report warning that the U.S. trade deficit is unsustainable. The IMF expressed concern that America's chronic trade deficit has increased the nation's net foreign debt to about 20% of its gross domestic product. Few countries have been able to carry foreign debts of that magnitude for long. Currency devaluation and an economic downturn are the usual result of such poor trade management, both of which would knock the props out from under American world leadership.
* The "trade deficit" is a myth. Doesn't exist.
* Protectionism CAUSES currency devaluation, because it hinders imports, which brings less US dollars into foreign hands, who have less US currency to spend on US goods, which brings a decline in the USD.