Lucius Cornelius Cinna (d.84 BC) was a leader of the opposition to Sulla, and helped overthrow Sulla's supporters after Sulla's first march on Rome, but was killed just before Sulla returned to Italy at the start of Sulla's Second Civil War.
We know very little about Cinna before his bid for the consulship in 87 BC. He is mentioned in Cicero's Pro Fonteio, where he is included in a list of men of Praetorian rank who commanded during the Social War. Livy mentions him as a commander against the Marsians alongside Metellus Pius, probably in 88 BC.
In 88 BC Sulla was serving as consul, and had been given command of the war against Mithridates VI of Pontus (First Mithridatic War). The elderly Gaius Marius also wanted the command, and he allied with Sulpicius, one of the tribunes of the plebs, to get Sulla's command transferred to him. Sulla's attempts to stop this failed and he was forced to flee from the mob. Sulla refused to accept his defeat, and convinced his army at Nola to march on Rome and take control of the city (battle of the Esquiline Forum, 88 BC). Sulla undid Sulpicius's laws, and regained his command. He may also have introduced a number of reforms, although these might also be dated to the aftermath of his second civil war.
Sulla saw himself as representing the legitimate government of Rome, and so despite his military victory he allowed the elections for 87 BC to go ahead as normal. His own candidates were defeated, and instead Cinna and Gnaeus Octavius were elected. Cinna was a known opponent of Sulla, and in an attempt to make sure that his reforms would survive once he was in the east, Sulla made the new consul-elect take an oath to support his policies. Cinna took a stone to the Capitol, and prayed that if he did not maintain his goodwill towards Sulla then he would be cast out of the city, just as the stone was thrown from his hand.
As soon as his own term as consul began in 87 BC Cinna broke this promise and appointed Virginius, one of the tribunes of the plebs, to impeach Sulla for his actions. At about the same time Sulla's fellow consul from 88 BC, Quintus Pompeius, was murdered while attempting to take command of Pompey Strabo's army, which was still in the field after the Social War. Sulla was either confident enough in the security of his reforms to ignore this or worried about his own safety, and set off for the east.
Cinna's next act was to try and gain support from the new Italian citizens, who had been granted citizenship as a result of the Social War. The new citizens had been allocated to eight new voting tribes, which would always be called to give their results last, meaning that their votes would rarely ever count. Cinna put forward a law to distribute the new voters in the existing voting tribes. In theory this would have allowed the numerous Italians to swamp the existing Roman voters, but in practice very few Italians would have been able to come to Rome to vote in person. Even so Octavius was able to gain the support of the old voters, and some of the tribunes of the plebs. On the day of the vote Cinna's supporters dominated the forum, and rioted after the tribunes vetoed the law. Octavius gathered a mob of his own, attacked Cinna's supports and drove them out of the city. Cinna attempted to save his position by offering freedom to any slaves who joined him, but this failed. The Senate declared that Cinna could be deposed as consul, and selected Lucius Merule, the priest of Jupiter, as his replacement.
Cinna didn't take his defeat lying down. Instead he began to raise an army from the Italian towns near Rome, and then won over an army that was at Capua (perhaps engaged in the ongoing siege of Nola, which had fallen to the Samnites during the Social War and was still in their hands). He was able to win over this army, and combined with his Italian troops this gave him a powerful force. He also had the support of a number of other aristocrats, amongst them Marius the Younger and the able Quintus Sertorius.
Octavius and Cinna didn't have the only armies in Italy. In some areas the Social War was still smouldering away, and some of the armies raised for that conflict were still intact. Octavius summoned one of those armies, under Pompey Strabo, to Rome, but after his arrival Stabo camped outside the city, and for some time it wasn't clear whose side he would take. Cinna decided to try and assassinate Pompey Strabo and his son (the future Pompey the Great), and managed to win over Pompey junior's tent mate Lucius Terentius. Strabo was a successful but unpopular commander, and the plan almost succeeded. Strabo was saved by his son, who discovered the plot, set a guard around his father's tent, and then managed to retain the support of his father's troops. After this Strabo joined Octavius and the defenders of Rome, but he died before the end of the siege. Soon after his death Rome surrendered to Cinna and Marius.
In his turn Cinna summoned Marius back from his exile in Africa. He arrived with another army, and Cinna and his supporters then besieged the city (siege of Rome, 87 BC). Octavius and Merule put up a better fight than Marius had managed in the previous year, but Cinna and Marius managed to cut off the food supplies to the city. An attempt to capture the Janiculum Hill, on the west bank of the Tiber, was repulsed, and this two armies then appear to have moved away from the city into the Alban Hills. This was a fatal mistake on the part of Octavius and his party, as in their absence they lost control of the Senate, which entered into peace negotiations. Cinna and Marius were allowed into the city. Octavius and his supporters retreated to the Janiculum, where Octavius was caught and beheaded. His head was taken to Cinna, and then displayed in the forum.
The fall of Rome was followed by a massacre of Cinna's and Marius's opponents. Amongst the dead were the father and brother of the triumvir Crassus, Gaius Julius Caesar Strabo Vopiscus and his half brother Lucius Julius Caesar, Atilius Serranus, Publius Lentulus, Gaius Nemetorius and Marcus Baebius. 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. An attempt to capture Sulla's wife and children failed, and they eventually managed to reach him in the east.
Unsurprisingly Cinna and Marius were selected as the consuls for 86 BC, but a few weeks into his seventh consulship Marius died. Cinna replaced him with L. Valerius Flaccus, who was then given the command of the war against Sulla in Asia. For the next few years Cinna dominated in Rome. He was consul again in 85 BC and 84 BC, this time alongside Gnaeus Papirius Carbo.
During his march on Rome in 88 BC Sulla had almost no support amongst high ranking Romans, but Cinna and his allies managed to alienate a great many of the leading men of the city, and enough of them fled to Sulla in the east to give him a sizable portion of the senate.
By 84 BC it was clear that Sulla was preparing to invade Italy, having ended the First Mithridatic War by making a peace treaty with Mithridates. Cinna and Carbo raised a army in Italy, and then prepared to ship it to the Balkans, to deal with Sulla before he could cross to Italy.
There are two different versions of Cinna's death. The first comes in Plutarch's life of Pompey. When Sulla was believed to be on his way back to Italy, the young Pompey decided to side with Cinna and went to his camp. He was clearly unpopular amongst Cinna's supporters, who will have remembered that his father had fought on the other side in 87 BC, and quietly withdrew after being accused of an unspecified offensive. A rumour spread around the camp that Cinna had killed Pompey, and this encouraged Cinna's opponents to rise against him. Cinna was chased by a centurion and attempted to buy his safety with his valuable seal ring. The centurion turned it down on the grounds that he wanted to 'punish a lawless and wicked tyrant', and killed him.
Appian tells a different story. The first detachment of troops safely crossed the Adriatic, but the second ran into a storm, and the survivors had to limp back to Italy. Once they were back on dry land they deserted. The rest of the army decided that it was no longer willing to cross to the Balkans just to fight other Romans. Cinna called them to an assembly to try and restore order. Faced with a large and angry mob, Cinna mishandled the situation. As he was approaching the assembly one of his lictors struck someone who was in his way. One of the mutinous soldiers struck the lector, and Cinna ordered the soldier's arrest. This angered the army, which turned on Cinna who was stabbed to death. This is probably the more likely story - at this point the young Pompey was still an unknown figure, so it seems unlikely that an army would mutiny because of him, but it is possible that the rumours about his fate had added to the angry mood of the army.
With Cinna gone Carbo was left as sole consul for the year. He cancelled the movement to the Balkans, and Sulla's Second Civil War (83-82 BC) would thus be fought on Italian soil.