Alaric belonged to the generation of Visigoths who, after Adrianople in 378, had settled inside the empire as federates. He had served in the Roman army, and repeatedly petitioned the western court for unpaid wages and for land — only to be stalled. When negotiations with the eastern court collapsed, he laid siege to Rome in 408 and again in 409; on the second occasion he starved the city until the Senate paid a heavy ransom. The emperor Honorius, safe in Ravenna, rebuffed every settlement, and so Alaric came back a third and final time.

On the night of 24 August 410 the Salarian Gate was opened from within — most likely by Gothic slaves inside the walls. The sack lasted three days. The contemporary sources (Jerome, Augustine, Orosius, Sozomen) describe a comparatively restrained destruction: the Visigoths were Christians and spared those who took refuge in the great basilicas, above all Saint Peter's and Saint Paul's; palaces were plundered and some buildings burnt, but the city was not levelled. Galla Placidia, half-sister of the emperor Honorius, was taken captive and would later marry Alaric's successor Athaulf. Alaric himself died in southern Italy within weeks; the Visigoths then turned toward Gaul and Spain.

The cultural shock far outweighed the physical damage. Rome — the 'Eternal City', untouched by foreign forces since the Gauls in 387 BCE — was no longer inviolable. For pagan Romans the cause seemed obvious: the empire had embraced Christianity, and the old gods had abandoned the city. It was to answer that charge that Augustine of Hippo began, in 413, to write 'De Civitate Dei' (The City of God), arguing that earthly empires are transient while the true 'city' is heavenly. The work was no mere polemic; it would become one of the foundational books of European political thought for the next millennium.

The sack of 410 did not end the Western Roman Empire — that would limp on for another sixty-six years. But it was the visible fracture point in a long erosion: in 455 the Vandals under Gaiseric would sack Rome a second time; in 476 Odoacer would depose the last western emperor; and the Gothic, Vandal and Frankish federate kingdoms that took shape after 410 in Gaul, Spain and North Africa would gradually become the independent Germanic monarchies of post-Roman Europe. Those three August days were the moment when contemporaries had to admit that an order they had thought eternal was not.