The History of the Decline and Fall of the Roman Empire: (Abridged, annotated)

According to Gibbon, the Roman Empire succumbed to barbarian invasions in large part due to the gradual loss of civic virtue among its citizens.

He began an ongoing controversy about the role of Christianity, but he gave great weight to other causes of internal decline and to attacks from outside the Empire.
"The story of its ruin is simple and obvious; and, instead of inquiring why the Roman empire was destroyed, we should rather be surprised that it had subsisted so long. The victorious legions, who, in distant wars, acquired the vices of strangers and mercenaries, first oppressed the freedom of the republic, and afterward violated the majesty of the purple. The emperors, anxious for their personal safety and the public peace, were reduced to the base expedient of corrupting the discipline which rendered them alike formidable to their sovereign and to the enemy; the vigour of the military government was relaxed, and finally dissolved, by the partial institutions of Constantine; and the Roman world was overwhelmed by a deluge of Barbarians."

-- Edward Gibbon. The Decline and Fall of the Roman Empire, Chapter 38 "General Observations on the Fall of the Roman Empire in the West"