While Hannibal was gaining the loyalty of the southern Spaniards, and organising their resources, Rome was occupied in trying to extend her power over the Gauls settled in the plain of the Po, and so to make sure of her northern flank, as she had already secured Sicily in the south. The Senate knew something of Hannibal’s design, and hoped to anticipate him in getting a hold on that valuable region, the strategical key to Italy. But here there was no question of gaining the loyalty of the tribes; the Gauls were restless and hostile, and had quite lately made another determined attempt to reach Rome; they actually came within three days’ march of the city before they were defeated in a great battle. In 219-218 B.C. Roman armies were still busy in driving roads northward, and planting two colonies, Placentia and Cremona, on Gallic soil and on the Po, when Hannibal descended on them from the Alps. He had found a pretext for war, gathered a force of 100,000 men, passed the Pyrenees and reached the Rhone before the Senate knew what he was about, and eluded a consular army dispatched to stop him. Scipio, its commander, with true military instinct, sent his army on to Spain, to cut his communications with the base he had been preparing so long. This line of communication Hannibal never recovered for ten years, and was forced to maintain and recruit his army on Italian soil.
That army, from a purely military point of view, was without doubt one of the best known to history. It consisted chiefly of thoroughly trained Spanish infantry, officered by Carthaginians, and of the best cavalry in the world, recruited from the Numidians of the western region of North Africa. It was one of those armies that can go anywhere and do anything at the bidding of its general, because entire trust in him was the one motive actuating it. It was a professional army, a perfect instrument of war, a weapon admirably fitted to destroy, but without constructive value—with no sap of civilisation giving it permanent vital energy. Luckily for Rome, this army had shrunk to very moderate dimensions when it reached Italy; the length of the march, the necessity of leaving some troops in Spain, and the terrible trials of the crossing of the Alps, where the native tribes combined with rock, snow and ice to wear it out, had reduced it to less than 30,000 men.