From March 537 to March 538 Belisarius held Rome against a much larger Gothic army under Vitiges, for a year and nine days.
Belisarius entered Rome in December 536 with a force of a few thousand, and in March 537 the Gothic army arrived to take it back.
He held the city for a year and nine days against far superior numbers, and the siege is among the most impressive defensive operations of the period.
It is also the moment Rome as an ancient city effectively ends: the aqueducts were cut and never properly restored, and the population that had depended on them left.
Rome had opened its gates to Belisarius without a fight, and the Gothic garrison had withdrawn north.
Vitiges came south from Ravenna with an army Procopius numbers implausibly high but which was certainly many times the size of the garrison.
Belisarius spent the interval repairing the Aurelian Walls, digging a ditch, and stockpiling grain — preparations that decided the siege before it began.
The Goths built camps around the northern and eastern approaches and assaulted the walls with towers and rams. The first great assault was beaten off with heavy losses.
They cut all the aqueducts, which ended the city's water supply, closed the public baths and stopped the mills — Belisarius replaced the last with floating mills moored in the Tiber.
The fighting included a defence of the mausoleum of Hadrian, where the garrison reportedly broke up the statues decorating it and threw the pieces down on the attackers. Belisarius counter-attacked constantly with small mounted sorties rather than waiting behind the walls.
The Gothic army wasted through the winter from disease, hunger and casualties, and withdrew in March 538 having achieved nothing.
Belisarius had held, but Rome had not survived intact. The aqueducts stayed broken, and a city that had already shrunk enormously since the fourth century now lost most of what remained.
The war moved north, and Rome would change hands four more times before it ended — each time with less of it left.
It is the defensive counterpart to Ad Decimum: a demonstration that a small professional force, well handled and well prepared, could hold against far greater numbers.
It also shows the shape of the whole Italian war. Belisarius won, and won brilliantly, with resources that should not have been enough — and the empire took that as proof it need not send more.
The cutting of the aqueducts is the detail that outlasts the campaign. The siege ended a system of urban life that had run for eight hundred years, and Rome did not have running water on that scale again for a millennium.