Nearly all western nations regarded themselves as heirs of the Roman Emire. Above all occidental art had its roots in Renaissance and therefore in Roman culture or in that what was taken for it. It was Roman civilasation where all European nations searched their patterns. In neoclassicism they tried to copy the architecture, political structures, livestyle and art. Shure that this where in many cases only imaginations, but Rome seemed to be the forefather of all.
But when most where satisfied to compare their modern empires with that of Rome, some started to think about decadence. The book of Edward Gibbon "The History of the Decline and Fall of the Roman Empire" became famous. And with the time people were less interested in the grandeur of the Roman Empire but in its decadence. Roman decadence became synonymical to Social criticism.
An appropriate expression of this accomplished the French artist Thomas Couture (1815-1879) with his painting "The Romans during the Decadence" (1847). Couture wanted to criticize his own contemporary french society. So he explained his painting with a quotation from the Roman poet Juvenal: "Crueller than war, vice fell upon Rome and avenged the conquered world".