This was the cause of the second Punic war - " declared by Rome and conducted, on the Carthaginian side' almost entirely by Hannibal". Encyclopaedia Britannica.
Following the first Punic war, Rome had gained naval superiority and the Carthaginian fleet had all but been dismantled. This control would have given Rome a choice over where the war would be fought, a choice Hannibal couldn't let them make. He recognised that an important part of Rome's power lay in the readily available resources of Italy, and saw that the way to defeating the republic lay in controlling and using these resources against the Romans instead. To this end, Hannibal left his brother Hasdrubal in command of a considerable army for the defence of Spain and North Africa, gathered his soldiers, and crossed the Ebro in April-May of 218, marching into the Pyrenees.
Polybius credits Hannibal's army with consisting of 90,000 infantry, 12,000 cavalry, and a number of elephants. Modern historians, however, seem to think that these figures are exaggerated and agree that a total force of about 40,000 is more likely.
The Pyrenean tribes met the Carthaginian forces with considerable resistance, and battles and desertions greatly diminished Hannibal's fighting troops - nevertheless, he managed to reach the Rhone river with little resistance from the tribes of southern Gaul. Meanwhile, the Roman general Publius Cornelius Scipio transported his army by sea to Massilia in order to intercept Hannibal. As Scipio moved northward along the right bank of the Rhone, he learned that Hannibal had already crossed the river and was marching northward on the left bank. Realizing that Hannibal probably planned to cross the Alps, Scipio quickly returned to northern Italy to await him.
~The Crossing~.
Crossing the Rhone with all his men, cavalry, and elephants, Hannibal was soon to embark on the trek which is probably the favourite and most famous of his movements in history.
Hannibal became commander of the Carthaginian army in 221. ... In 219, Hannibal captured Saguntum, an ally of Rome. War with Rome was inevitable and 218 saw the beginning of The Second Punic War. ... Hannibal would later be exiled and Carthage would never regain its former power. ... Rome would never again be threatened until the empire's fall hundreds of years later. ...
Hannibal became commander of the Carthaginian army in 221. ... In 219, Hannibal captured Saguntum, an ally of Rome. War with Rome was inevitable and 218 saw the beginning of The Second Punic War. ... Hannibal would later be exiled and Carthage would never regain its former power. ... Rome would never again be threatened until the empire's fall hundreds of years later. ...