by Edward Gibbon
According to Gibbon, the Roman Empire succumbed to barbarian invasions because of a loss of civic virtue among its citizens. They had become lazy and soft, outsourcing their duties to defend their Empire to barbarian mercenaries, who then became so numerous and ingrained that they were then able to easily take over the Empire. Romans, he believed, had become effeminate, unwilling to live the military lifestyle.
"In the Roman Empire, the word barbarian meant anyone not a Roman citizen, which was applied chiefly to northern European tribes outside of the influence of Roman culture."
Here we go.