[3] Although sailing to Italy might have been faster and free from the hazards of a land march, at sea the entire fleet might be lost in a storm,[4] and Roman naval dominance[5][note 1] increased the risk to Hannibal's large armada,[note 2] as it risked suffering crippling losses from Roman naval attacks despite Carthaginian warship escorts.

[14] Hannibal needed to time his movements carefully to keep the Romans in the dark, for if the Romans got hint of his intentions, they had enough recourses to fight a multi-front war:[20] they could send one army to block the Carthaginian army at the Pyrenees, station a strong force in Cisalpine Gaul, and invade Africa with another simultaneously; or stand on the defensive with overwhelming forces in North Italy.

[21] After the successful conclusion of the Siege of Saguntum, Hannibal did nothing to provoke the Romans; he dismissed his army and did not immediately march for Italy in the Spring of 218 BC after he received the news of war.

[33][note 4] Hannibal left Cartagena probably in late May or early June,[34] timing his departure to allow the spring flooding of Spanish rivers to subside,[35] ensuring the availability of food and fodder along the way,[36] and after receiving envoys from Gallic tribes from the Po valley, who assured him of their willingness to cooperate against the Romans.

As Atillius neared Tannetum, the Gauls retired without battle,[65] and the Romans spent the summer of 218 BC recovering and fortifying Placentia and Cremona, probably in a two month long operation.

Hannibal's passivity and non-threatening defensive dispositions during March – May[32][24] probably influenced this decision by reinforcing the Roman perception they were fighting a repeat of the first war and the initiative lay with them.

[72] The area bordered Rhoda and Emporiae, colonies of Roman-allied Greek Massalia, so some of the neighboring tribes may have been pro Roman due to their influence.

[35] Hannibal spent the months of July and August of 218 BC crossing the two hundred miles from the Ebro River to the Pyrenees, conquering the area by campaigning against the “Illurgetes” (perhaps not the Ilergetes at Lérida who were pro Carthaginian, but probably another obscure tribe between Tarraco and Barcino),[73] the Bargusii at Serga valley, the Ausetani between Ausa (Vich) and Gerona along with the Lacetani, the Aeronosii, and the Andosini tribes.

[74] Hannibal stormed a number of unspecified cities and this campaign aimed to subdue region as quickly as possible, leading to heavy Carthaginian casualties.

A general named Hanno, who has been identified by various authors as Hannibal's nephew[78] (son of Hasdrubal the Fair), a brother,[79] or no Barcid relation, garrisoned the newly conquered territory north of the Ebro with 10,000 infantry and 1,000 cavalry, and based himself specifically to watch over the Bargusii, a pro-Roman tribe.

[74] The trans-Ebro campaign allowed Hannibal to give his new recruits valuable combat experience, and impress upon the Iberian tribes the extent of the power of Carthage, especially those which might have been influenced by the Greek colonies of Massalia,[85] which were not captured,[86] and his shedding of unwilling troops and heavy baggage allowed him to form a more mobile, streamlined, loyal, battle-hardened experienced army,[87][73] also reduce his supply and provisioning burden by decreasing the number of soldiers, pack animals and size of the baggage train.

[91] Hannibal's negotiation skills and war chest were put to good use to placate individual Gallic tribes on the way,[10] so the foraging of the Carthaginians caused no friction; supplies may also have been purchased from the Gauls as no reports exist of any fighting taking place during his march.

[92] Hannibal was opposed by some Gallic tribes at the Rhône, and lost days defeating the Gauls, which enabled Scipio to locate him by pure chance[93] when he landed with his army at the allied Greek city of Massilia in mid September.

[63] Scipio had already known before setting sail that Hannibal had crossed the Ebro, and assumed that the Carthaginians were still engaged beyond the Pyrenees,[94] so he disembarked his troops, made camp, unloaded his heavy baggage, and allowed his soldiers to recuperate from their sea voyage.

Scipio decided against following Hannibal into the Alps, as his army was not equipped and provisioned for a winter campaign, and marching in unknown territory risked being ambushed by Gauls or the Carthaginians.

After marching back to the seacoast, Scipio decided to send the bulk of his forces to Iberia under the command of his elder brother Gnaeus (who had been consul in 221 BC), while he himself returned to Northern Italy with a small escort to organize the defenses against Hannibal.

[73] Hanno probably started raising Iberian levies,[109] and Indibilis, chieftain of the Ilergetes who also held sway over the Lacetani and Sussateni tribes,[111] probably commanded these troops.

[102] Hasdrubal Barca, who was in Cartagena nearly 300 miles away,[112] was caught off guard by the sudden arrival of the Romans,[113] and marched north to join Hanno with an army of 8,000 infantry and 1,000 cavalry.

Hasdrubal, arriving too late to aid Hanno and not being strong enough to attack the Romans, still crossed the river and sent a flying column of light cavalry and infantry on a raid.

[120] The Ilergetes began to raid pro-Roman tribes, and Hasdrubal may have crossed the Ebro again in support, forcing Scipio to send punitive expeditions and restore peace.

After punishing the officers in charge of the naval contingent for their lax discipline, and distributing the captured booty among his soldiers, Scipio and the Roman army wintered at Tarraco.

[1][note 8] The Gallic rebellion delayed the arrival of Scipio by two to three months[70] and foiled Hannibal's strategic goal of securing Spain, but even as Hannibal trimmed down his army by reducing its size and leaving his heavy baggage at Cissa to gain mobility and march towards the Rhône, he still expected to meet the Romans in Gaul, and kept the size of his army big enough to deal with the Romans.

Scipio's decision to send his brother to attack Spain mirrors the strategy Memnon of Rhodes had advocated against Alexander the Great to the Persians.