Hannibal caught the Romans on the wrong foot by eluding their forces in France and by emerging so quickly from the Alps, in about October 218 BC.
Previously, on realising that he had missed catching Hannibal, Publius Cornelius Scipio (the father of Scipio Africanus), sailed back to Italy and rushed to the Po River to protect the recently founded Roman colonies of Placentia (modern Piacenza) and Cremona.
Most sources suggest that Hannibal started his journey from Spain with 50,000 men. After evading the Romans and pushing through some unfriendly Gallic tribes whilst crossing the Alps probably about half of these were still present. After recruiting additional men among the friendly Insubres, a Gallic people of northern Italy, to compensate for the loss of about 15,000 men during the long march, Hannibal subjugated the Taurini, a tribe hostile to the Insubres. He then forced into alliance with himself all the Ligurian and Celtic tribes on the upper course of the Po River. His troop strength was raised to about 40,000.
The army of Publius Cornleius Scipio now moved to hold a strong position on the Ticino River with two legions and some allied Gauls. Here he waited for Tiberius Sempromius Longus to reinforce him with 2 more legions. It was on this river than Hannibal's skilled cavalry tactics were to give Rome its first bloody nose in what was to be called the battle of Ticinus