The siege of Rome took place on 2 March 537 – 12 March 538 AD and it was the city's first siege during the Gothic War (535–554) between the defending Byzantine Empire's forces under the leadership of General Belisarius against a numerically superior Ostrogothic (Goths) force under King Vitiges. The siege was the first major encounter between the forces of the two opponents, and played a decisive role in the subsequent development of the war.
Belisarius prepared the fortification of Rome for the upcoming siege and organized the city's defense with about 5,000 troops, installed artillery on the walls, and recruited citizens to assist in manning the walls. The Goths established fortified camps around Rome and attempted to cut its water supply. They soon launched an assault against Rome on multiple fronts using siege towers, battering rams, and ladders, but the defenders repulsed the Gothic attacks.
The failure of the Gothic assault turned the siege into a prolonged struggle of attrition. Gothic forces captured the port of Portus and attempted to further isolate Rome. The food shortages and disease increasingly affected the population, who placed pressure on Belisarius to act. Byzantine reinforcements eventually reached the city, enabling Belisarius to make extensive use of cavalry, including mounted archers, in sorties against the besiegers. After an unsuccessful attempt to defeat the Goths in open battle, Belisarius largely relied on the city's fortifications, cavalry hit-and-run tactics to effectively besiege the besiegers. A temporary armistice gave the Byzantines time to strengthen their defenses and increase pressure against the Goths, who also had food shortages issues. The siege ended in March 538 after Byzantine forces under John the Sanguinary captured Ariminum (modern-day Rimini), a strategic city near the Gothic capital of Ravenna. Vitiges abandoned the siege on 12 March and withdrew north with his army to capture Ariminum.
In the late 5th century, the Italian Peninsula came under the control of the Goths, who, although they continued to acknowledge the Byzantine Empire's suzerainty. Theodoric the Great (r. 493–526) sought the creation of an independent kingdom by balancing the Gothic military dominance with the preservation of Byzantine civil institutions, fostering cooperation between the senatorial elite in Rome and the Gothic ruling class. After Theodoric's death in 526, his daughter, Amalasuintha, attempted to continue pro-Byzantine policies and maintain close ties with Byzantium under Emperor Justinian I. However, she kept her position as regent until her son came of age through the assassination of political opponents, and relying on Byzantine advisers, and her son had a Byzantine education. These policies alienated segments of the Gothic nobility. Following her son's death in 534, the opposition revived the question of succession, culminating in Amalasuintha's deposition and murder by Theodahad in 535. Justinian considered this as a casus belli justification to intervene militarily in the Italian Peninsula; framing his campaign as both a defense of a legitimate ruler and a restoration of imperial authority, he launched a military campaign that became known as the Gothic War.
In 535, the Byzantine general Mundus invaded Dalmatia, and Belisarius, with an army of 7,500 men, captured Sicily with ease. From there, the following June, Belisarius crossed over to Italy at Rhegium. The Byzantines captured Naples after a twenty-day siege in early November, and the city was sacked due to their resistance. After the fall of Naples, the Goths, who were enraged with the inactivity of their king, Theodahad, gathered in council and elected Vitiges as their new king. Theodahad, who fled from Rome to Ravenna, was murdered by an agent of Vitiges on the way. In the meantime, Vitiges held a council at Rome, where it was decided not to seek immediate confrontation with Belisarius, but to wait until the main army, stationed in the north, was assembled. Vitiges then departed Rome for Ravenna, leaving a 4,000-strong garrison to secure the city. He also had to address a crisis of legitimacy due to the lack of royal blood, which forced Vitiges to strengthen his claim. He traveled to Ravenna and, after divorcing his wife, forced a marriage to Matasuintha, the only surviving child of Queen Amalasuintha and the granddaughter of Theodoric.
Vitiges then proceeded to negotiate with the Franks, who were in alliance with the Byzantines. Theodahad had proposed handing over the Gothic territory in Gaul, along with 2,000 pounds of gold, in return for their agreement to assist him in the war. Theodahad died before the transaction was concluded and Vitiges sought to complete this arrangement. The Frankish kings agreed but they were unwilling openly to break their alliance with the Emperor Justinian. Instead, they secretly promised to send warriors drawn from their tributary peoples to maintain plausible deniability.
The citizens of Rome, unwilling to support the risks of a siege in the light of the sack of Naples, supported the Byzantines. A delegation on behalf of Pope Silverius and eminent citizens was sent to Belisarius. The Goth garrison quickly realized that, with the population hostile, their position was untenable. On 9 December 536 AD, Belisarius entered Rome through the southern Asinarian Gate at the head of approximately 5,000 troops, while the Gothic garrison was leaving the city through the northern Flaminian Gate and headed north towards Ravenna. After 60 years, Rome was once again in the hands of the Empire. Belisarius settled in Rome, preparing for the inevitable counterstrike. He set up his headquarters on the Pincian Hill, in the north of the city, and started repairing the walls of the city. He also placed torsion ballista at towers and stone throwers (onagri) along the parapet of the wall. Grain was arriving daily from Sicily and was stored in the granaries around the city. A ditch was dug out on the outer side and a chain was drawn across the Tiber River. A number of citizens conscripted to assist in the defense of city and the food supplies brought from Sicily were stored in granaries, which were under guard. The populace of the city was becoming increasingly aware that a siege was inevitable.
The Gothic army marched on Rome and gained passage over the River Anio at the Ponte Salario (Salarian Bridge), where the defending Byzantines abandoned their fortifications and fled. The next day, the Byzantines were barely saved from disaster when Belisarius, unaware of his forces' flight, proceeded towards the bridge with a detachment of his bucellarii. Finding the Goths already in possession of the fortified bridge, Belisarius and his escort became engaged in a fierce fight, and suffered casualties before extricating themselves. On the same day, Vitiges sent his commander Vacis to the Salarian Gate to remind the citizens of Rome of the treachery to the Goths for preferring the protection of the "Greeks" (i.e., Byzantines), who had not been in Italy except as "actors" and "thieving mariners". His speech was ineffective and the Goths started preparations for the siege the following day.
The Goths set up seven camps, overlooking the main gates and access routes to the city. Six of them were east of the river, and one on the western side, on the Campus Neronis, near the Vatican Hill. This left the southern side of the city open. They proceeded to block the aqueducts that were supplying the city with its water, necessary both for drinking and for operating the gristmills. The mills were those situated on the Janiculum, and provided most of the bread for the city. Although Belisarius was able to counter the latter problem by building floating mills on the stream of the Tiber River, the hardships for the citizenry grew daily. Perceiving this discontent, Vitiges tried to achieve the surrender of the city by promising the Byzantine army free passage, but Belisarius refused the offer, as follows: