While Hannibal was making his march across the Alps, the Romans took the fight and retaliation for Saguntum, directly to the Carthaginians in Spain. An invasion by a Roman Consular army under Publius Cornelius Scipio was launched in 218 BC, but a revolt among the Celts in Cisalpine Gaul forced a change in the plans. P. Cornelius Scipio returned to Italy to deal with the revolt and the impending arrival of Hannibal, while his brother Gnaeus Cornelius Scipio, took the invasionary army on to Hispania. Gnaeus had under his command an initial force numbering 22,000 infantry, 2,200 cavalry and a strong fleet of 60 quinqueremes.
Gnaeus Scipio landed at Emporiae in NE Hispania, in October 218 BC and immediately advanced south, taking control of territory as far as Tarraco. Uncontested by Carthaginian resistance as he marched, he set to work subjugating local Iberian Celts. Hannibal's brother, Hanno, left in command in Northern Spain, decided to meet Scipio despite commanding a far inferior force. Outnumbered by as many as 2 to 1, Hanno's army was crushed near the town of Cissa, and Hanno himself was captured. As a result and from the very outset of the Roman invasion, Rome was able to secure a port as a supply base and also immediately nullify Spain as a source of supply and reinforcement for Hannibal in Italy.
In 217 BC, Hasdrubal, now in command of the Carthaginian forces, recruited heavily among the local Iberians. His fleet was brought to a strength of about 40 ships under Himilco. He advanced on Scipio's position on the Ebro River with his combined ground and naval forces, but his fleet was caught completely by surprise by a recently reinforced contingent of Roman ships. With the victory on the Ebro, the domination of the Roman navy was never again challenged throughout the length of the entire war. Shipping lanes and Carthaginian ports were blockaded and controlled which would eventually have a significant impact on Hannibal's campaign in Italy.
After the victory at the Ebro, the Roman senate sent Publius Scipio back to Spain with reinforcements of 8,000 men. Gnaeus raided the Balearic Islands to put down a revolt of the local Iberians and Publius took control of the overall navy. In the year 216 BC, both Roman and Carthaginian commands were occupied consolidating control over their own territories rather than fighting one another. The Romans grappled with King Indibilis and his Balearic Iberians, and Hasdrubal with the Tartesii tribe. Because of the Tartesii, Hasdrubal, despite recent reinforcements of 4,000 infantry and 500 cavalry from Africa, had to postpone any plans to check the Roman advance until the following year.
With the opening of the campaign season in 215 BC, Hasdrubal Barca led his army of 30,000 north to meet the Romans. The Scipios meanwhile, with a comparable force moved south to block Hasdrubal at the Ebro. At the small town of Detrosia, the armies met in very similar conditions to those that unfolded for Hannibal at Cannae. Hasdrubal's plan all along was to mimic Hannibal's strategy and hold the Roman infantry in the center while his cavalry enveloped the flanks. Unlike Hannibal's army, Hasdrubal lacked the disciplined cavalry of his brother and the result was a far different outcome. Quickly after the battle opened, the Scipios recognized the strategy and effectively countered it. In the end, Hasdrubal's army was routed and its effects were felt throughout the course of the entire war.
The defeat at Dertosia was monumental for the Carthaginians. Many local Iberian tribes shifted their allegiance to Rome, and control of the vast mineral wealth of Spain was slowly crumbling. The Romans seized several cities south of the Ebro and took control of territory belonging to the Carthaginian allied tribes, the Intibili and the Illiturgi. The arrival of Mago with 12,000 infantry, 1,500 cavalry and 20 elephants helped to avert complete disaster for Carthage, but its effect on Hannibal in Italy was profound. Mago had been enroute to join Hannibal and the diversion helped stem the Roman advance in Hispania but reduced the overall effectiveness of the Italian campaign.