The outbreak of the Second Punic War began when Hannibal moved north across Ebro to begin his historic march over the Alps. Before leaving Spain, however, Hannibal was well aware that Roman forces intended for him would try to meet him there. He secured Spain with an army of about 16,000 men under the command of Hasdrubal and took 80,000 infantry, 12,000 Numidian and Iberian cavalry and a number of elephants with him on his march.
Early in the spring of 218 BC, Hannibal set out from Carthago Nova, for the Ebro River. He was confidant in Hasdrubal's ability in Spain as his brother had campaigned with his father Hamilcar against the Iberian Celts since he was just a boy. Unfortunately for Hasdrubal, Hannibal took all the senior command and elite troops with him on his march, which would play a role late in the war. Hasdrubal's duty was to maintain Carthaginian dominion over Spain and to defend the primary interests (especially mines and resources) from Roman countering forces. With success in those primary goals, he was to raise an additional army and follow Hannibal to Italy.
After crossing the Ebro in April or May of 218 BC, Hannibal had little choice to conquer local tribes as he moved. Leaving a violent population in his rear could've been disastrous, and despite the time delay and cost in casualties of operations against the Celts, he rapidly subdued the area. Except for the Greek coastal cities, which leaned towards Rome in diplomatic alliances, all of Spain was secure. The general Hanno was left with 10,000 infantry and 1,000 cavalry to keep the area between the Ebro and the Pyrenees under control.
With his rear secured, Hannibal continued north. Allowing some native Spanish troops to return to their homes (and possible transfer to Hasdrubals army) and deductions for Hanno's occupation force, he continued on with 50,000 men, 9,000 cavalry and his elephants. Later in the same year, 218 BC, Hannibal marched through the Pyrenees and into Gaul, never to return to Spain.
Once in Gaul, Hannibal was met with only slight resistance from native Celts there. Along the march from the Pyrenees to the Rhodenus (Rhone) Hannibal's original planning started to bear fruit. The Gallic Celts were no friends of the Romans and many joined with him while en route. According to Polybius, they crossed the Rhone at a distance of a 4 days march from the sea, using boats made by local Celts for the infantry and cavalry and large, flat earth covered wooden skiffs for the elephants.
During the crossing of the Rhone, Hannibal and his army finally started to meet some resistance. Gauls appeared on the opposite bank to disrupt the crossing, but Hannibal was ready. A force under Hanno was sent further upstream to cross and attack the Gauls in the rear. The successful maneuver ended the threat, and peaceful crossing resumed. With the hostile Celts disposed, only friendly tribes remained on either side of the Alps and Rome's only chance to stop Hannibal was to meet him at the Rhone.
Two Roman armies had been raised for the purposes of dealing with Carthage. The first, commanded by Publius Cornelius Scipio the Elder, was set to depart for Spain. The second, under the command of T. Sempronius, was originally intended to be an invasion force in Africa. Hannibal's approach, among other factors, inspired a revolt among the Boii and Insubrian tribes in Cisapline Gaul, and Roman plans were forced to change. Scipio's army was circumvented from going to Spain and sent with Lucius Manlius to defend the Po Valley from the Gauls. Sempronius' forces were already in Sicily preparing for the African invasion, and Scipio had to wait in Rome for another army to be raised before he could meet Hannibal marching east.
It wasn't long before another army was ready for Scipio. He and his brother Gnaeus Cornelius Scipio sailed to Massilia, in southeastern Gaul, with the intention of meeting Hannibal before he could reach the safety of the Alps. Hannibal, however, had outmaneuvered the Romans and was already moving northward along the Rhone, with the intention of moving around the Romans and then south through the mountains. The Romans knew they had no choice of stopping Hannibal's march and had to react. The army of Sempronius was brought up to the Po, and his invasion of Africa was scrapped. Scipio the Elder, returned to Italy to await the Carthaginians, while the bulk of his army went west with Gnaeus Scipio into Spain.
Upon his escape from the Roman roadblock, Hannibal moved into Transalpine Gaul territory occupied by the Boii. Knowing the local Celts relationship with Rome, Hannibal took full advantage. The Celts were eager to help Hannibal cross the Alps, and their aid, knowing the safe passages, likely was a major factor in his successful march through them. Prior to going through the mountains, however, Hannibal's army was under supplies and exhausted after marching 750 miles from Carthago Nova to Transalpine Gaul. A civil war being fought between two brothers of the same undetermined tribe in a very fertile region in the mountain foothills also worked in Hannibal's favor. In exchange for helping secure his position, the tribal chief fed the Carthaginians and provided enough supplies to see them through the rest of the journey. By October of 218 BC, Hannibal and his menacing force were ready to cross the Alps into Italy.