Spartacus: leader of an army of runaway slaves that shook Italy

Gladiators on a relief from the first century BCE. Glyptothek, München (Germany).
Spartacus: leader of an army of runaway slaves that shook Italy in 73-71 BCE. He was defeated by the Roman general Crassus.
The Roman economy was based on agriculture and war. For centuries, a Roman citizen was a peasant and a soldier. During the Second Punic war (218-202; against the Carthaginian general Hannibal), this started to change. The Romans had to fight their wars overseas: in Hispania, and, after 200, Greece and Macedonia. Often, the soldiers had to stay abroad for a long time, and it often happened that on their return, they found that their farms had gone bankrupt. Under these circumstances, there was only one solution: sell the farm and move from the country to the city.
The Italian cities were rapidly growing, and the countryside also changed.Slowly, the small farms were replaced by large plantations (often called latifundia), where the work was done by slaves, who could not be recruited for military service. The Greek historian Appian of Alexandria (c.95-c.165) describes the results:
The rich [...] used persuasion or force to buy or seize property which adjoined their own, or any other smallholdings belonging to poor men, and came to operate great ranches instead of single farms. They employed slave hands and shepherds on these estates to avoid having free men dragged off the land to serve in the army, and they derived great profit from this form of ownership too, as the slaves had many children and no liability to military service and their numbers increased freely. For these reasons the powerful were becoming extremely rich, and the number of slaves in the country was reaching large proportions, while the Italian people were suffering from depopulation and a shortage of men, worn down as they were by poverty and taxes and military service. And if they had any respite from these tribulations, they had no employment, because the land was owned by the rich who used slave farm workers instead of free men.
[Appian, Civil wars 1.7; tr. John Carter]In this way, the countryside became crowded with slaves: usually prisoners of war, but often simply bought from slave traders, who bought them from pirates. (A modern estimate: there were two million slaves on an Italian population of six million.) Strong captives were sometimes forced to fight as gladiators in the arena. The ancients really loved this bloody spectacle, something we could expect from the bellicose Romans (although gladiatorial contests were just as popular in the Greek world).
One of those was Spartacus, the leader of a rebellion of gladiators and slaves that escalated to a full-scale war in the years 73-70. We have two main sources: Plutarch of Chaeronea (46-c.122) describes this war in his Life of Crassus (text), and one generation later, Appian told the story in his History of the Civil wars (text). Both accounts describe more or less the same events in exactly the same sequence, and it is tempting to see the same source behind their stories, probably the Histories of Sallust or (less likely) Livy's History of Rome from its foundation. It seems that Appian has abridged his account, whereas Plutarch has left out several stories about Spartacus' cruelty.
In 73, seventy-eight gladiators managed to escape from the fighting school of Gnaeus Lentulus Batiatus at Capua. According to Plutarch, they were only armed with choppers and spits, which they had found in a kitchen. However, they soon discovered a transport of gladiatorial weapons. From now on, they were heavily armed, and they occupied a mountain.
Appian informs us that this was the Vesuvius, and that the gladiators elected three leaders: Spartacus, Oenomaus and Crixus. Probably, they represented ethnic groups: a Thracian, a Greek, and a German. According to Plutarch,
Spartacus was a Thracian from the nomadic tribes and not only had a great spirit and great physical strength, but was, much more than one would expect from his condition, most intelligent and cultured, being more like a Greek than a Thracian.
[Plutarch, Life of Crassus 8; tr. Rex Warner]This last remark is a well-known cliché from ancient literature. Any non-Greek/Roman who had done something special, was said to be more intelligent than other barbarians. Other sources say that Spartacus could have so much success because he had once fought in the Roman auxiliaries.
Already at this stage of the revolt, runaway slaves, shepherds, and herdsmen must have joined the band of gladiators (our sources mention this at a later stage). We have to assume this, because otherwise, it is impossible to explain how the gladiators were able to overcome a militia sent by the Capuan authorities to deal with the runaways. The only result was that the gladiators now had real arms. Their numbers quickly swelled, because, as Appian tells us, Spartacus "divided the spoils in equal shares".
The central government at Rome now had to intervene, and it sent the propraetor Gaius Claudius Glaber with an army of 3,000 hastily conscripted and untrained soldiers. Perhaps this was an underestimation of the power of the gladiators, but it is more likely that Rome was unable to send a stronger force. The empire was involved in two large wars: general Pompey was fighting against Sertorius in Hispania and his colleague Lucullus against king Mithridates VI of Pontus in the east. The city itself was restless because, due to these wars, grain had become scarce.
Related articles Plutarch on Spartacus Appian on Spartacus Florus on Spartacus Livy, Periochae,95, 96, 97

A Roman magistrate and two lictors carrying fasces (©!!)
Although he had a small and untrained army, Claudius came close to success. He isolated the gladiators on a hill-top which was covered with vines, and it looked as if they were chanceless. However, the besieged made ladders from the branches of the vines, descended from the hill during the night, and managed to get behind the enemy lines. The Romans panicked and fled, and their camp was looted by the gladiators. They could start to give weapons to the runaway slaves who had joined them.
"Rome" launched a second expedition against the gladiators, this time commanded by the praetor Publius Varinius. For reasons that are unknown to us, he divided his forces, and the divisions were easily defeated by the army of the gladiators. Varinius himself was humiliated: he lost the very horse that he rode, his lictors were taken prisoner, and Spartacus paraded their fasces through his camp.

The Roman author Publius Annius Florus, who published a summary of the great History of Rome from its foundation of Livy, mentions that the army of gladiators and slaves "laid waste Nola, Nuceria, Thurii and Metapontum with terrible destruction" (text). These towns are all situated in the southern half of Italy. The shepherds of this region, real cowboys, joined the army of Spartacus. From now on, he could also employ cavalry.
Next year, the Senate understood that this war was serious. According to Appian, Spartacus now commanded some 70,000 people, and although we do not know how he obtained this figure, we can be sure that the wealthy land-owners in the Senate understood that their slaves could also run away. Therefore, the senators ordered both consuls, Lucius Gellius Publicola and Gnaeus Cornelius Lentulus Clodianus, to proceed against the bands of Spartacus.