The following year in 146 BC, the Romans sent Praetor Caius Plautius to Spain with 10000 fresh infantry troops and 1300 cavalry.
Upon his arrival, Viriathus quit Carpetania and withdrew into Lusitania whereby Plautius sent an advanced force of about 4000 to pursue him. Utilizing his signature 'feigned flight' tactic, Viriathus turned on his pursuers and routed the Romans, killing most in the process.
Plautius and the rest of his forces made haste and crossed the Tagus to avenge the slaughter and met Viriathus on an olive-tree covered mountain top known as the Hill of Venus; where he was encamped.
There, Viriathus defeated Plautius' army so roundly that the Roman general and the remainder of his forces withdrew into winter quarters even though it was still midsummer. For the rest of the year, Viriathus' forces overran the country without check and levied new resources by threatening the destruction of crops in Roman administered territories.
Once news of the latest developments reached Rome, the Senate deemed the situation threatening enough to send a consul and consular army into Spain. Correspondingly, the next year Quintus Fabius Maximus Aemilianus, the adopted son of Lucius Aemilius Paulus, conqueror of Macedon was given Spain as his province. In an effort to spare the worn out veterans of the Punic, Greek and Macedonian wars, Aemilianus levied two legions of mostly new recruits and headed for Spain.
Aemilianus and his army did not arrive until summer of 145 BC which indicates that they perhaps made their way by sea, instead of overland through Gaul and thus had to wait for the sailing season to commence. He mustered his and some allied forces in the town of Urso in Hispania Baetica, near Astapa. Not wanting to face Viriathus until his inexperienced army had been sufficiently drilled, he left the army in the care of his legate and went to Gades to offer a sacrifice to Hercules at the famous Temple of Melqart.
During Aemilianus' absence Viriathus attacked the Roman foragers, killing many and terrifying the others. Using poor judgment, the legate came out to fight Viriathus and was defeated swiftly while Viriathus was able to capture standards and booty. When Aemilianus returned, Viriathus consistently attempted to draw the Romans out onto the battlefield but Aemilianus was never tempted. He continued to exercise his troops, only sending them out on minor skirmishes in an effort to strengthen their resolve, test the enemy's strength and give them much needed experience. It was also from this point on that the foragers went out only with an armed cordon of troops as Aemilianus had seen his father do in Macedon.
The next year at the end of winter quarters, Aemilianus was continued in his post by the Senate and he deemed the army sufficiently ready to fight. Over the course of 144 BC the Romans succeeded in putting Viriathus' forces to flight but only after numerous valiant struggles. Regardless, Viriathus was on the run and the Romans succeeded in capturing two of Viriathus' chief towns after which Aemilianus and his army went into winter quarters in Corduba.
Regardless of the successes of Aemilianus, Viriathus' campaign against the Romans had inspired many Celtiberian tribes to follow his example. In 143 BC, Celtiberia broke out in an open insurrection that would later be known as the Numantine War which more than neutralized the Roman's current good fortune against Viriathus.