The siege of Rome (87 BC) saw the opponents of Sulla seize control of the city after a prolonged campaign in the vicinity of Rome. The fall of the city to Cinna and Marius was followed by a massacre of Sulla's supporters and the real or imagined enemies of Marius, who died soon after beginning his seventh consulship (Sulla's First Civil War).
In 88 BC Marius and the Tribune Sulpicius had attempted to outmanoeuvre Sulla, who was then serving as one of the Consuls for the year, using the popular assembly to pass a series of laws, including one to more fairly distribute the new Italian voters amongst the existing 35 voting tribes and one to give Marius command of the war against Mithridates VI of Pontus, replacing Sulla. The plan failed after Sulla fled to his army and convinced it to march on Rome. The resulting battle of the Esquiline Forum was the first time Roman soldiers had fought inside the walls of the city for at least 400 years. Sulpicius was captured and killed, while Marius fled into exile in Africa.
Sulla's attack and reforms weren't popular in Rome, and his candidates for the consulship of 87 BC were both defeated. The consuls for 87 BC were Lucius Cornelius Cinna and Gnaeus Octavius. Octavius proved to be a loyal defender of Sulla's reforms, but Cinna was a dedicated opponent. As soon as his year of office began he attempted to prosecute Sulla. Sulla was already focusing on the eastern war, and decided to leave Italy with his army and leave Octavius and his other supporters to deal with the problem. At first this policy appeared to be successful. Cinna put forward a law to restore Sulpicius's voting reforms, and tried to push it through with the support of an armed mob. The majority of the Tribunes attempted to veto the law, but they were overwhelmed by the mob. At this point Octavius intervened with his own armed supporters, and after a mass brawl Cinna was forced to flee from the city.
After this things began to go wrong. There were at least two Roman armies still active in Italy. On the Adriatic coast Pompey Strabo (father of Pompey the Great) retained the command of the army he had led during the Social War. To the south of Rome Appius Claudius Pulcher commanded an army based at Capua. After being expelled from Rome, Cinna fled to Capua, where he gained control of Claudius Pulcher's army (either by bribery or by portraying himself as the wounded party in the disputes at Rome). He then moved around Italy, gaining more troops from the recently enfranchised Italian tribes, who were alienated by Sulla's conservatism. He was said to have raised 300 cohorts, a powerful force.
The most detailed narrative of the resulting campaign comes from Appian. Livy and Plutarch provide some extra details, but their accounts generally support Appian's framework.
While Cinna was raising an army, Octavius and his new colleague Merula repaired the walls of Rome, built trenches and placed engines on the walls. They summoned supporters from loyal towns in Italy and from Cisalpine Gaul. They also summoned Pompey Strabo to Rome. Strabo marched across the Apennines and camped outside the Colline Gate, at the northern tip of Rome, but at first he appears to have offered his support to both sides. Cinna responded by advancing towards Rome and camping near Strabo. Strabo was a successful but unpopular general, and it was probably at this stage that Cinna attempted to assassinate Pompey senior and Pompey junior and seize control of his army. Pompey junior's tentmate, Lucius Terentius, was bribed to kill Pompey and then set fire to Strabo's tent. He would then seize control of the army. Young Pompey learnt of the plot, and coolly dealt with it. After pretending to retire for the night he snuck out of this tent, placed a guard around his father's tent and waited for Terentius to act. He repeatedly stabbed the empty bed, and his action triggered a wider revolt throughout the camp. Strabo remained in his tent, unwilling to risk facing his troops, but the younger Pompey moved amongst the troops and slowly won all but 800 of the back to his father's cause. After this Strabo sided firmly with Octavius.
Cinna now gained fresh supporters of his own. News of his revolt had reached Gaius Marius, in exile in Africa. He gathered a force of around 1,000 men (a mix of exiles, African cavalry and freed slaves), and crossed over to Telamon in Etruria, 85 miles to the north-west of Rome. He judged his audience superbly. He promised freedom to the slaves, gathered an army of 6,000 Etruscans and a fleet of 40 ships, and advanced south towards Cinna, still looking unkempt from his time in exile and with his hair uncut, to make a point about how he had been treated. Cinna offered to make Marius a pro-consul and sent him the insignia of office, but Marius turned them down. This display of humility wasn't supported by his actions.
Marius joined Cinna outside Rome. Their army was split in three in order to blockade the city. Cinna and Carbo were posted 'opposite' the city - presumably facing the walls facing away from the Tiber. Marius was posted between Rome and the sea, and Sertorius above the city on the Tiber. Sertorius and Marius build bridges across the river to prevent food supplies reaching the city. Marius then took part of his army to the coast and sacked the port of Ostia.
In an attempt to gather more troops, the Senate ordered Q. Caecilius Metellus Pius to make peace with the Samnites and bring his troops back to Rome. Metellus refused to accept the Samnites's terms, and they decided to ally with Cinna and Marius instead. They wouldn't have much impact on the events of 87 BC, but they did play a part in Sulla's Second Civil War. At about the same time Cinna sent troops to occupy Ariminum (Rimini), to stop any troops from Cisalpine Gaul joining the Sullans.
This was followed by the main fighting of the siege. The defences of the Janiculum, on the west bank of the Tiber, were in the hands of the military tribune Appius Claudius. He owed Marius a favour, and agreed to open a gate for him at daybreak. Marius let Cinna in, and all four of the Marian commanders then took part in an attack on the Janiculum. Octavius and Pompey Strabo were able to drive them out of the city, although at some cost. Soon after this Strabo died, either of the plague that was ravaging his camp, or when his tent was struck by lightning. He was so unpopular that his body was dragged from its bier on the way to the funeral pyre, and had to be rescued.
Although the sources don’t explicitly state it, at this point the Marians appear to have at least partly lifted the siege and instead concentrated on cutting off Rome's food supplies. Marius used his fleet to stop the import of food by sea and up the Tiber. He then captured Antium, Aricia, Lanuvium and other local towns where grain for Rome was stored. By the time this part of the campaign was over, the Marians were some way from Rome, and approached the city along the Appian Way. Marius, Cinna, Carbo and Sertorius then stopped 100 stades from the city (about 11 miles). Their opponents also left the city, and Octavius, Metellus and P. Licinius Crassus (the father of the triumvir) took up a position at the Alban Mount, a similar distance from the city. They probably had the stronger army, but were unwilling to risk the fate of Rome on a single battle.
This proved to be a fatal error. Cinna sent heralds around offering freedom to any slave who would desert to him, and a sizable number took up the offer. This alarmed the Senate, which sent envoys to Cinna to negotiate for peace. Octavius had lost control of events in the city by leaving, and would never get it back. Cinna responded to the first envoys by asking if they came to him as a private citizen or a consul. They had no immediate answer, and had to return to the city. The number of deserters now began to increase, partly because the effective blockade meant that food was increasingly scarce and partly because Marius and Cinna appeared to have the upper hand. Cinna advanced closer to Rome and camped just outside missile range. The Senate decided to depose Lucius Merula, and treat with Cinna as a consul. They asked him to refrain from bloodshed, but he refused to take an oath to that effect. He did promise that he would not willing be the cause of anyone's deaths, but he did warn Octavius, who had circled around the city and entered by one of the western gates, to avoid the forum as he couldn't guarantee his safety. Marius was said to have stood silently by Cinna's chair, making it clear by his appearance that he was planning a bloody revenge.
The Senate gave in, and invited Cinna and Marius into the city. Although they were invited in, they treated the city as if it had been conquered. Octavius retreated to the Janiculum with his remaining allies, and refused to flee. His party was attacked by cavalry led by Gaius Marcius Censorinus, and Octavius was beheaded. His head was then displayed by the rostra in the forum, the first time this had happened to a consul. He would soon be joined by the heads of those senators who were killed in the bloodbath that followed.
The fall of Rome was followed by a massacre of Cinna's and Marius's opponents. Amongst the dead were Gaius Julius Caesar Strabo Vopiscus and his half brother Lucius Julius Caesar, Atilius Serranus, Publius Lentulus, Gaius Nemetorius and Marcus Baebius. Crassus (father of the triumvir) killed his younger son and then killed himself. Marcus Antonius, the grandfather of the triumvir, took shelter in a farm, but was discovered and killed when the farmer sent a slave out to buy better quality wine than normal. Lucius Cornelius Merula, Cinna's temporary replacement as consul, committed suicide just before he was due to go on trial, as did Quintus Lutatius Catulus, Marius's colleague at the end of the Cimbric War.
Cinna and Marius appointed themselves as the consuls for 86 BC, fulfilling a prophecy that Marius would be consul seven times. He didn’t live long to enjoy his success, dying in mid-January 86 BC. Cinna remained in power in Rome for several years, but was killed by his own troops when attempting to put down a mutiny just before Sulla returned from the East in 83 BC (Sulla's Second Civil War). However Cinna had managed to set up a fairly stable regime which survived his death, and Sulla found it much harder to win power in 83-82 BC than he had in 88 BC.