I am listening to a lecture series on Rome and the Barbarians and Professor Harl has spent three lectures discussing the Roman conquest of Spain. I was not familiar with these activities and found it fascinating. Professor Harl said the Celt-Iberians were skilled warriors that were recruited by both the Romans and Carthaginians to fight in the Punic War. He pointed out, however, that after Carthage was defeated, the Celt-Iberians were not kept in the regular Roman Army for service elsewhere so they continued to raid the former Punic towns as a way of supporting themselves. He mentioned that they also suffered pressure from overpopulation in an area that could not readily support large numbers. The end result was that the Romans had to turn around and fight these former allies to protect the towns that were now under their control.
Professor Harl said these wars in the mid-2nd century B.C.E. were particularly brutal, with casualties in the 40% range, because the Celt-Iberians were not only skilled but had been trained in sophisticated Roman and Carthaginian tactics. He said eventually Rome committed over 100,000 men to the control of Spain, primarily because of its mineral wealth. He also mentioned that Spain became the target of many ambitious Roman commanders for "triumph hunting". He cited cases where subsequent governors would intentionally start wars by breaking existing treaties just to try to wangle a triumph out of the senate.
I also thought it was interesting that during the Republican period, Sertorius used mythology to help control these Spanish peoples by having a pet fawn appear at propitious times engendering the belief that he was favored by the goddess Diana.