On this day, December 17th, 546 Totila, king of the Ostrogoths conquered the city of Rome. During the siege he destroyed the aqueducts that had made life in the eternal city viable. After 546 the population dropped rapidly, buildings finally fell into ruin and in the eyes of many ancient Rome finally fell.
Totila did not set out to destroy the ancient Roman way of life. The Ostrogoths found themselves in a struggle for survival against the (Eastern) Roman Emperor Justinian. The Ostrogoths had occupied Italy since 493 with approval from the emperor in Constantinople. They pursued a policy of live and let live with the local Roman population. The Ostrogoths counted no more than 100,000 individuals whilst Italy at that time still had a population of several million. To manage their new kingdom, they kept key institutions like the Roman Senate and maintained at least some of the infrastructure.
After their great leader Theoderic (475-526) had died his succession became contested and as a consequence Ostrogothic power was weakened. The emperor Justinian sought to use this weakness to regain the western parts of the Roman empire. The first Gothic war lasted from 535 to 540 and ended with a defeat of the Ostrogoths. Shortly afterwards the Justinian plague broke out in the Eastern Roman empire and would ultimately kill as many people as the more famous Black Death of 1348.
From 541 onwards a faction of the Ostrogoths resumed hostilities hoping that Justinian would be weakened by the outbreak of the plague. By 546 the new leader of the Ostrogoths, Totila, had recovered sufficient military might to attack the city of Rome itself. The siege was extremely brutal and Totila forced the surrender by destroying the aqueducts that lead water into the city of Rome. Totila could not hold on to his conquests for long. He was defeated in 552 by the great Byzantine general Narses. His successor Teja took an (almost) last stand on Mount Vesuvius a few months later where the last Ostrogothic army was destroyed.
The Byzantines were not much luckier. Northern Italy was soon overrun by the Lombards and Sicily by the Muslims, confining them to Ravenna and parts of Southern Italy.
After the destruction of the aqueducts the ancient Roman infrastructure of the city could no longer be maintained. The last of the great baths closed and without ready access to drinking water the population of the city shrinks from what might still have been 100,000 before the siege to maybe 20,000.