Sulla's First Civil War (88-87 BC) was triggered by an attempt to strip him of the command against Mithridates and saw Sulla become the first Roman to lead an army against the city for four hundred years. Although he was able to regain the command, his political setup in Rome collapsed almost as soon as he left Italy, and the war would resume in 83-82 BC (Sulla's Second Civil War).
In 89 BC war finally broke out between Rome and Mithridates VI of Pontus (First Mithridatic War, 89-85 BC). L. Cornelius Sulla, one of the consuls for 88 BC and an experienced military leader who had served under the famous Gaius Marius, was given the command of the war. Eastern wars were famously lucrative for their commanders, and there was also a great deal of prestige to be gained. The elderly Marius also wanted the post, despite having had to retire from the field after the first year of the Italian Social War. Marius had already had one encounter with Mithridates, when he warned him not to fight against Rome, and clearly felt that the command was already his. By this point the early working relationship between Marius and Sulla had broken down, and the two men were increasingly bitter rivals.
The crisis came during 88 BC. Sulpicius, one of the Tribunes of the Plebs for the year wanted to get the recently enfranchised Italian voters split fairly through the existing 35 voting tribes, so that their votes would actually count. After the Social War the new voters had been placed into eight new tribes, and as the results of votes were announced in order of precedence their votes would hardly ever count, as most issues would have been decided before the last eight tribes were reached (the existing voters feared being swamped if their tribes were filled with the new voters, although in practice very few of the new Italian citizens would have been able to spare the time to come to Rome to vote on any regular basis). The Senate blocked Sulpicius, so he allied with Marius, and put forward a list of proposals to the popular assembly, which in theory was the sovereign power in Rome. These included the voting law and the change of command. Normally the Senate gave out military commands, although that wasn't officially one of its powers, and the assembly had been used to alter a command before, but it was still a radical move. Sulla reacted by retiring to examine the heavens for omens, one of his rights as Consul. This meant that all public business had to stop until the Consul completed his examination. Sulpicius responded by bringing out the Roman mob in his support (a increasingly common tactic in Roman politics). Sulla only escaped by taking shelter with Marius, and he had to agree to allow public business to resume. The law passed, officially passing the eastern command to Marius.
At this point Sulla broke with all precedent. He escaped from Rome, and managed to reach the army at Nola before Marius's military tribunes. Sulla was able to win the soldiers over to his side, and Marius's tribunes were killed when they attempted to take control. This was a key moment in the slow fall of the Roman Republic. For the first time a Roman army had chosen to side with its commander against the Republic. Although Sulla's motives were generally conservative, and he saw himself as a defender of the Republic, he set a very dangerous precedent.
Sulla now led his six legions towards Rome, the first Roman to lead troops against the city since the possibly legendary Coriolanus 400 years earlier. He was joined by his fellow consul Quintus Pompeius Rufus, and their army quickly broke into the city. A battle followed at the Esquiline Forum, and after a promising start Marius and Sulpicius were overwhelmed. Sulpicius was betrayed by a slave and executed (as was the slave), while Marius managed to escape from the city, and after a series of adventures reached safety in Africa.
After the battle Sulla advanced to the Via Sacra, in the heart of the city, and posted troops around the city. He and Pompeius then spent the following night making sure that no disorders broke out - it wouldn't have suited Sulla's view of himself as the defender of the Republic if his troops had looted the city. On the next day he addressed the popular assembly to justify his actions and put forward a number of constitutional reforms. Sulla believed that powers of the popular assembly were at the heart of the political instability in Rome. In order to deal with this Sulla proposed that no laws could be passed by the popular assembly unless it had already been discussed by the Senate, and that voting should be by the centuries, where the wealthiest had more power, and not by the tribes, where every vote was equal. He also wanted to reduce the power the tribunes, and added 300 new members the Senate. Sulpicius's laws were annulled on the grounds that they had been imposed by violence…
Soon after this Sulla sent his army back to Capua, and reclaimed his post as consul. This encouraged his political opponents to begin to agitate for the recall of the exiles, and against Sulla and Pompeius. There was also still another army active in Italy, under the command of Gnaeus Pompeius Strabo (the father of Pompey the Great). In an attempt to secure Pompeius Rufus's position, Sulla had command of this army transferred to him. When Rufus arrived at his new army, Strabo pretended to give up command, but on the following day a crowd of soldiers murdered Rufus. Strabo pretended to be angry, but also resumed control of the army.
Sulla's control of Rome wasn't at all secure. In the elections for 87 BC his candidates (Nonius and Servius) were rejected in favour of Lucius Cornelius Cinna and Gnaeus Octavius. Cinna was a known opponent of Sulla, and in an attempt to limit his freedom of action forced Cinna to make an oath to support his policies.
Cinna almost immediately broke his oath. Soon after coming into office he attempted to impeach Sulla, and found a tribune, Virginius, who was willing to accuse him. Sulla was already committed to his eastern command, and ignoring Cinna's provocation departed for Greece to face Mithridates. Once Sulla was gone, Cinna attempted to revive Sulpicius's voting plans, with the support of the new citizens. Octavius led the opposition, supported by the old citizens. On the day of the vote Cinna's supporters occupied the Forum, and then began a riot when the Tribunes vetoed the law. Octavius led his supporters to the forum, and managed to defeat Cinna's men. Cinna fled from the city, leaving Octavius in sole command. Octavius deposed Cinna, and replaced him with Lucius Cornelius Merula. This was a blatantly illegal action, and probably helped gain Cinna more respectable supporters.
Cinna managed to win over a Roman army at Capua, and raised a force from the Italians. Marius returned from exile in Africa, and they besieged Rome. Octavius gained the support of Pompey Strabo (father of Pompey the Great), who helped repel a serious assault on the city, but died soon afterwards. The Marians then concentrated on cutting off the food supply to Rome, and the action moved away from the immediate area of the city. The two armies faced off around the Alban Mountains, to the south-east of Rome, but Octavius then lost control of the Senate, which entered into negotiations with Cinna and Marius. They were eventually allowed into the city without any further fighting. Octavius refused to flee and was beheaded, and his head displayed in the Forum.
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.
Sulla was declared a public enemy, his house was burnt down and his property was confiscated. His laws were repealed, and Cinna and Marius made themselves consuls for 86 BC. Marius died in mid January, a few days after entering into his seventh consulship, and after staining his reputation with the bloodbath that followed the fall of the city. Cinna managed to establish a fairly stable regime, and served as consul for the next few years. In the meantime Sulla continued with his campaign against Mithridates, and after a series of successes expelled him from Greece and reoccupied the lost Roman provinces. He then made peace with Mithridates, and prepared to return to Italy. Cinna was killed while trying to put down a mutiny amongst troops who were unwilling to risk the sea voyage to the Balkans to face Sulla there, but his successors managed to put up a surprising credible, if eventually unsuccessful, resistance to Sulla (Sulla's Second Civil War).
Sulla's attack on Rome was a turning point in the history of the Republic. Before then Rome hadn't seen hostile soldiers within the walls for centuries. Afterwards civil war almost became the normal state of affairs, with conflicts in 83-82 (Sulla's Second Civil War), Lepidus's Revolt in 78-77 BC, the Sertorian War in Spain for most of the 70s, a brief period of peace in the 60s and 50s, then the Great Civil War of 50-44 BC and the prolonged civil wars that followed the death of Caesar. By the time Octavian took power a stable Republic was a distant memory