In 154 BC, the Roman senate objected to the Belli town of Segeda building a circuit of walls, and declared war.
The next consul, Lucius Licinius Lucullus, attacked the Vaccaei, a tribe living in the central Duero valley which was not at war with Rome.
The classical sources put the blame of starting the Second Celtiberian War on the city of Segeda (near Zaragoza).
Appian wrote that the war broke out because this powerful city of the Celtiberian tribe of the Belli persuaded the people of some smaller towns to settle there and was building a circuit of walls seven kilometres long.
The Belli had agreed to the treaties Tiberius Sempronius Gracchus had made with tribes in Hispania at the end of the First Celtiberian War.
It forbade the building of the wall, demanded the tribute and the provision of a contingent for the Roman army in accordance with the stipulations of Gracchus' treaty.
[2] Classical sources also comment on other movements and seditions by other cities in the Celtiberian territories, and grave problems in Hispania Ulterior, where Punicus and Caesarus headed a Luso-Vettonic coalition against Rome.
Expecting a long war in Hispania, the Senate decided in 153 BC for the first time that the election of the magistrates would take place on 1 January, instead of 15 March.
This allowed Quintus Fabius Nobilitor to arrive in Hispania and start his campaign early in the year.
The people of Segeda, whose wall had not been completed, fled and sought refuge among the Arevaci of Numantia (7 km north of Soria), who welcomed them.
[4] The Arevaci assembled at the town of Numantia which had strong natural defences, and chose Ambo and Leuco as their leaders.
These Roman disasters encouraged the town of Ocilis (Medinaceli, also in the modern province of Soria) to defect to the Celtiberians.
[5] In 152 BC Marcus Claudius Marcellus, consul for the third time, took over the command, bringing 8,000 infantry and 500 cavalry to Hispania.
His moderation encouraged the people of Nertobriga (a town of the Belli, in the modern province of Zaragoza) to ask for peace.
Marcellus chained the horsemen, sold their horses, plundered the countryside and began to besiege the town, which sent a herald to ask for peace again.
The Nertobriges sent ambassadors to these tribes and asked Marcellus for leniency and for the renewal of the treaty made with Gracchus.
Marcellus sent envoys from each party to Rome to carry on their dispute there and sent private letters to the senate urging peace.
Because of this their envoys were admitted into the city, while those of the Arevaci, as they were enemies, were ordered to encamp on the other side of the River Tiber.
They asked either that the Roman army should remain in Hispania and that it should be commanded by a consul to check the depredations of the Arevaci or, if the troops were to be withdrawn, that Rome should inflict an exemplary punishment on them.
According to Polybius, when the envoys of the Arevaci were heard, they came across as not being willing to submit or to accept defeat and gave the impression that they thought that they had fought more brilliantly than the Romans.
[7] Appian wrote that the senate was not happy that these people had refused the terms put forward earlier by Nobilitor.
Then, the young Publius Cornelius Scipio Aemilianus spoke in the senate and asked to be allowed to be sent to Hispania as an officer or a junior commander and that he was ready to assume such role.
He sought to persuade the Celtiberians to put matters in his hands because he wanted to bring the war to an end before the arrival of Lucullus.
[11] Appian wrote that Lucius Licinius Lucullus was greedy for fame and money and attacked the Vaccaei because he was ‘in straitened circumstances'.
The Caucaei attacked a party of Roman wood cutters and foragers, killed many of them and pursued the fugitives to their camp.
Appian wrote: "he, like all guilty souls, being angry with his accusers instead of reproaching himself, laid waste their fields".
The enemy did not respond, save for one man who often rode into the gaps between the Roman armies and challenged them to single combat.
When some of the siege works were completed the Romans knocked down a section of the city walls, but the attackers were quickly overpowered.
In 144 BC, the fourth year of this war, Viriathus, the Lusitanian leader, incited the Celtiberians to rebel.