This is of course the legendary crossing of the Alps in an attempt to establish a base of power in Italy.
Hannibal was joined by friendly Gallic leaders and their men, whose knowledge of the passes of the mountains was an added bonus to their fighting power. At first danger came at them from behind, the Allobroges attacking them from the rear of the column. Along the length of the route hostile Celtic tribes would attack the baggage handling animals, or roll large boulders down on the troops from the heights also causing men and animals to panic and loose their footing on the icy ledges. The countermeasures Hannibal had to employ to keep the tribes at bay caused huge amounts of casualties. The march was delayed while a landslide had to be cleared to allow them passage, and the hardened ice from the previous year's snow combined with the currently occurring fall created terrible conditions for walking and many died from exposure.
Finally after 15 days, and with only 20,000 infantry, 6,000 cavalry, and a few of the original 38 elephants surviving, Hannibal descended into Italy. His troops had been ill fitted to undertake such a trek and now were totally inadequate to match the army of Scipio, who had rushed to the Po river to protect the recently founded Roman colonies of Placentia and Cremona.
~The Initial Victories~.
In spite of this, the first battles which Hannibal conducted have earned him fame as a great tactician. In the face of overwhelming opposition, the invading forces still managed to defeat the Romans on many occasions.
"The first action between the two armies took place on the plains west of the Ticino River, and Hannibal's Numidian cavalry prevailed. Scipio was severely wounded, and the Romans withdrew to Placentia. After manoeuvres failed to lead to a second engagement, the combined armies of Sempronius Longus and Scipio met Hannibal on the left bank of the Trebia River south of Placentia and were soundly defeated (December 218).
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. ...