1 America – a country that has leapt from
barbarism to
decadence without touching civilization. John O'Hara and he was an American.
2 America has progressed from barbarism to decadence without the intervening stage of civilization. George Bernard Shaw
3 America is the only country that went from barbarism to decadence without civilisation in between. Oscar Wilde.
Numerous other people have been credited with expressing that sentiment or something similar including
Henry James, another American. However, in all probability the phrase originates with Georges Clemenceau when he was President of France.