As in the times of the utmost danger, Rome once more called out twenty-three legions.  Volunteers were summoned to arm, and those legally exempt from military service were included in the levy.  Nevertheless, they were taken by surprise.  Far earlier than either friends or foes expected, Hasdrubal was on the Italian side of the Alps (547); the Gauls, now accustomed to such transits, were readily bribed to open their passes, and furnished what the army required.  If the Romans had any intention of occupying the outlets of the Alpine passes, they were again too late; already they heard that Hasdrubal was on the Po, that he was calling the Gauls to arms as successfully as his brother had formerly done, that Placentia was invested.  With all haste the consul Marcus Livius proceeded to the northern army; and it was high time that he should appear.  Etruria and Umbria were in sullen ferment; volunteers from them reinforced the Phoenician army.  His colleague Gaius Nero summoned the praetor Gaius Hostilius Tubulus from Venusia to join him, and hastened with an army of 40,000 men to intercept the march of Hannibal to the north.  The latter collected all his forces in the Bruttian territory, and, advancing along the great road leading from Rhegium to Apulia, encountered the consul at Grumentum.  An obstinate engagement took place in which Nero claimed the victory; but Hannibal was able at all events, although with some loss, to evade the enemy by one of his usual adroit flank-marches, and to reach Apulia without hindrance.  There he halted, and encamped at first at Venusia, then at Canusium:  Nero, who had followed closely in his steps, encamped opposite to him at both places.  That Hannibal voluntarily halted and was not prevented from advancing by the Roman army, appears to admit of no doubt; the reason for his taking up his position exactly at this point and not farther to the north, must have depended on arrangements concerted between himself and Hasdrubal, or on conjectures as to the route of the latter’s march, with which we are not acquainted.  While the two armies thus lay inactive, face to face, the despatch from Hasdrubal which was anxiously expected in Hannibal’s camp was intercepted by the outposts of Nero.  It stated that Hasdrubal intended to take the Flaminian road, in other words, to keep in the first instance along the coast and then at Fanum to turn across the Apennines towards Narnia, at which place he hoped to meet Hannibal.  Nero immediately ordered the reserve in the capital to proceed to Narnia as the point selected for the junction of the two Phoenician armies, while the division stationed at Capua went to the capital, and a new reserve was formed there.  Convinced that Hannibal was not acquainted with the purpose of his brother and would continue to await him in Apulia, Nero resolved on the bold experiment of hastening northward by forced marches with a small but select corps of 7000 men and, if possible, in connection with his colleague, compelling Hasdrubal to fight.  He was able to do so, for the Roman army which he left behind still continued strong enough either to hold its ground against Hannibal if he should attack it, or to accompany him and to arrive simultaneously with him at the decisive scene of action, should he depart.