Titus Livius (Livy), The History of Rome, Book 27
Cyrus Evans, Ed.

39. A letter which was brought out of Gaul from Lucius Porcius, the praetor, increased the alarm at Rome. [2] It stated that Hasdrubal had quitted his winter quarters, and was now crossing the Alps; that eight thousand Ligurians had been enlisted and armed, which would join him when he had crossed over into Italy, unless some general were sent into Liguria to engage them with a war. That he would himself advance as far as he thought it safe with his small forces. [3] This letter obliged the consuls hastily to conclude the levy, and go earlier than they had determined into their provinces, with the intention that each should keep his enemy in his own province, and not allow them to form a junction or concentrate their forces. [4] This object was much aided by an opinion possessed by Hannibal; for although he felt assured that his brother [p. 1145]would cross over into Italy that summer, yet when he recollected what difficulties he had himself experienced through a period of five months, first in crossing the Rhone, then the Alps, contending against men, and the nature of the ground, he was far from expecting that his transit would be so easy and expeditious, [5??] and this was the cause of his moving more slowly from his winter quarters. [6] But all things were done by Hasdrubal with less delay and trouble than he himself or any others expected. For the Arverni, and after them the other Gallic and Alpine nations in succession, not only gave him a friendly reception, but followed him to the war; [7] and not only had roads been formed during the passage of his brother in most of the countries through which he marched, and which were before impassable, but also as the Alps had been passable for a period of twelve years, he marched through tribes of less ferocious dispositions. [8] For before that time, being never visited by foreigners, nor accustomed, themselves, to see a stranger in their country, they were unsociable to the whole human race. And at first, not knowing whither the Carthaginian was going, they had imagined that their own rocks and forts, and the plunder of their cattle and people, were his objects; [9] but afterwards, the report of the Punic war with which Italy was being desolated for now ten years, had convinced them that the Alps were only a passage, and that two very powerful nations, separated from each other by a vast tract of sea and land, were contending for empire and power. These were the causes which opened the Alps to Hasdrubal. [10] But the advantage which he gained by the celerity of his [11??] march he lost by his delay at Placentia, while he carried on a fruitless siege, rather than an assault. [12] He had supposed that it would be easy to take by storm a town situated on a plain; and the celebrity of the colony induced him to believe that by destroying it he should strike great terror into the rest. [13] This siege not only impeded his own progress, but had the effect of restraining Hannibal, who was just on the point of quitting his winter quarters, after hearing of his passage, which was so much quicker than he expected; [14] for he not only revolved in his mind how tedious was the siege of towns, but also how ineffectual was his attempt upon that same colony, when returning victorious from the Trebia.