As a fact the Roman army had not the courage to go out and give battle since they were afraid of the enemy's cavalry, but they remained in their camp with complete confidence since they well knew that the cavalry to which they had owed their defeat in the battles could do them no harm there.
The Carthaginians again obviously could not remain there longer encamped together with their cavalry, since the Romans had with this very object destroyed all the forage in the neighbourhood, and it was impossible to get carried up from such a long distance enough hay and barley for so many horses and mules;
nor again if they remained in their position without their cavalry were they bold enough to assault an enemy having the advantage of protection by a trench and palisade, an engagement with whom on equal terms would be attended with doubtful success now they were deprived of their cavalry.
Besides this they were in dread of the consuls designate appearing and establishing themselves in their rear, and thus placing them in great difficulties by cutting off their supplies.
For these reasons Hannibal thought it would be impossible to raise the siege by force of arms and changed his plan.
For he thought that if by a secret march he could appear suddenly before <a class="linkToPlace" target="_blank" href="/place?placename=Rome&groupId=935&placeId=1669">Rome</a>, he might possibly by the surprise and dismay he would cause among the inhabitants manage to gain some advantage against that city itself;
or if not would at least compel Appius either to raise the siege and hasten to the help of his native town, or to break up his army, so that both the force that went to relieve <a class="linkToPlace" target="_blank" href="/place?placename=Rome&groupId=935&placeId=1669">Rome</a> and that which was left behind would be easy to overcome.
Walbank Commentary