The peace which Viriatus had managed to arrange with the Romans after his previous victories over their armies in Iberia, was not to last long. Technically upon the attempted assassination of Viriatus in 139 BC, the war had already begun and movements had taken place in Iberia. Caepio himself was the brother of Servilianus who had first made the treaty with Viriatus and a major opponent of such a peace treaty. This was why, when he had first arrived in Iberia, he had appealed successfully to the senate for permission to harass Viriatus’ army in an attempt to provoke the Iberian to break the treaty and attack them. Who exactly it was that broke the treaty in 139 BCE is unknown but what is known are the movements that took place early on into the war. Viriatus was well aware that he was massively outnumbered by Caepio and chose to go on the retreat while taking up a policy of scorched earth to prevent Caepio being able to properly supply his army. It was this retreat that allowed Caepio to take the town of Arsa early on into the war but also allowed Viriatus to escape his pursuers. Having lost Viriatus into Western Iberia, Caepio began to turn on the local people who had supported the Iberian during the conflict over the previous 8 years. It was at this point that Caepio finally turned to trying to assassinate Viriatus, recognising that the general was the primary reason why the Iberians had been able to resist the Romans so well and why the Romans had proven unable to subdue the Lusitani.
But with the assassination attempt on Viriatus’ life a failure and the Iberian still very much alive to fight against the Romans and resist their attempted conquest. Ironically this assassination attempt could only backfire upon its failure, in Rome this was a massively unpopular move and deemed cowardly by the people. While, in the short term, the underhanded harassment of Viriatus’ army remained a secret in Rome, the very attempt on the life of Viriatus was largely condemnable in Rome. This caused problems for Caepio whose very presence in Iberia was out of a desire to achieve a triumph for subduing Viriatus and now suddenly found his popularity in Rome starting to drop. In Iberia the assassination attempt of the man who had become a hero for many Iberians for his successful defiance of Rome, proved to be even more dangerous to the Romans. In combination with the harsh treatment of the local Vettones and Callaici tribes in the region around Arsa, the Romans suddenly found themselves dealing with a rippling effect of dissent from the Iberians throughout the region. Even in established areas of Roman Spain the Romans started to find that they lost a lot of support from the Iberians while Viriatus succeeded in swelling his numbers from local supporters. Caepio now found himself in a very precarious position in Iberia and the threat of an uprising in Eastern Iberia cutting off his supply lines would have been a disaster. His hopes to isolate the Lusitani and cut off Viriathus from supplies now seemed to be backfiring as he found himself in a much more precarious position in Iberia. In the North the Romans were facing a number of defeats under Quintus Pompeius in the Numantine war as the Roman positions steadily became more and more precarious.
In comparison Viriatus found himself in a much better position than before, previously he had found himself steadily becoming more isolated and cut off from Northern support, but no longer. His forces swelled from locals joining him against the Romans and refugees fleeing the capture of Arsa and the ravaging of local tribes there. But Viriathus was a cautious man and had no intention of fighting Caepio head on, instead he intended to turn back to guerrilla-style warfare against the Romans. His first step was to move North against a Roman force under the command of Marcus Popilius Laenas, a consul and governor of Hispania Citerior in 139 BCE. Marcus was moving South against Viriatus through the Duero Valley, a natural defence point against invasions. It was here that Viriatus hoped to stop Marcus and re-establish communication and support to the Numantines. His force was still relatively small and further depleted by him leaving about 2000 men to shadow Caepio and keep him under threat. This left Viriathus with anywhere between 5000-9000 Lusitani soldiers to defeat an army of about 10,000-15,000 under Laenas in the North. He moved quickly, hoping to be able to prevent Laenas from clearing the Duero Valley and, with such a small force, managed to reach the valley exceptionally quickly.
The Duero Valley is a region dominated by the Douro River but containing a number of tributaries, it’s made up of a number of small canyons and forms an ideal place to counter and defeat invasions. In early 138 BCE, Viriathus finally came into contact with Laenas’ army in the Duero Valley in a number of very minor skirmishes between the two forces. In these early skirmishes Laenas’ force succeeded in driving back Viriathus’ warriors and gained a number of minor victories. But after these initial skirmishes on the banks of Duoro River, Viriathus turned back to guerrilla warfare to defeat Laenas once and for all. One night in March 138 BCE Laenas finally caught up to Viriathus’ force as he saw a large group of about 3000 Lusitani soldiers moving South along the river. Desperate to catch up to the Iberians to end Viriatus’ threat once and for all, Laenas hurried his soldiers along and pursued the Iberian force. However this pursuit would last all night with the Iberians always remaining just ahead of the Roman force until, early the next morning, the Iberians finally turned to face the exhausted Romans. But before Laenas could even hope to line up his army for battle, Viriathus’ well-rested and energised ambush force swept down from the West. Caught between two forces, exhausted and taken completely by surprise, the Romans began to panic and a number of them broke and fled. However for the other Roman soldiers the battle took a very heavy toll on their forces, the sheer weight of the Iberian assault pushed their disorganised army back into the river and their bombardment by the Iberian slingers only forced them further back. Unsurprisingly the cavalry was the first to go as they were pushed back into the river, unable to free themselves and drowned. The heavily armoured infantry didn’t fare much better and was heavily weighed down by their armour and weaponry with those unable or unwilling to dump their equipment also being drowned. Those who managed to remain on the banks of the river were cut down by the Lusitanian fighters and the army torn apart by their enemy. Only a few days later a fisherman along the river found Marcus Pompilius Laenas’ lifeless body floating down the river.
The destruction of Laenas’ army along the Duoro River would allow Viriathus to regroup his army and make preparations to defeat Caepio and reclaim Arsa. While Viriathus had been fighting Laenas in the North, the 2000 fighters left in the South had been shadowing Caepio’s army. Throughout the months between Caepio’s capture of Arsa and the defeat of Laenas’ army in March 138 BCE, Caepio had made a number of attempts to push further against the Lusitani but to no avail. Every time he moved significantly beyond Arsa, Caepio found his supply lines being attacked and significantly threatened by Viriathus’ force. Beyond this the army had consistently shadowed and followed the Roman army which had left Caepio’s soldiers on edge. A large part of this came from the fact that Caepio and his soldiers had no idea where Viriathus was and, for all they knew, that force shadowing them was Viriathus and was a much larger force than it looked. This fear in conjunction with the attacks on his supply lines, left Caepio trapped in Arsa and unable to make a significant advance. But things were about to take a turn with a number of developments that were to finally put Viriathus on the back foot.