The rebels held the hills to the west of Carthage and the only bridge across the Bagradas river leading to Utica. Hamilcar observed that wind blowing from a certain direction uncovered a sandbar at the river mouth that was fordable and, under cover of night, the Punic army left Carthage and crossed the river. Hamilcar aimed to attack the small rebel band holding the bridge, but Spendius led the rebel force besieging Utica to confront Hamilcar.The Carthaginian army was caught in a pincer movement, Hamilcar pretended to retreat, and Spendius likely attempted to trap the outnumbered Carthaginians against the river with his two forces, pinning them with one and out-flanking them with the other. When his troops rushed towards the retreating Carthaginians Spendius was either unable to control them, or also believed that the Carthaginians were fleeing and encouraged his forces' pursuit. Hamilcar had managed to train his new recruits in some drill and basic battlefield maneuvers before they left Carthage. As the two rebel forces came into clear sight the Carthaginians wheeled, and marched away. The Carthaginians were marching in good order, so they can perform a pre-planned manoeuvre which they had practiced in Carthage, but the rebels, many of whom were inexperienced soldiers, believed that the Carthaginians were running away. Shouting encouragement to each other they broke into a run to pursue.Hamilcar unleashed his trap as the disorderly rebels closed on his formation. As the cavalry and then the elephants came closer to the infantry Hamilcar ordered each in turn to also wheel about to face the rebels. The modern historian Dexter Hoyos stresses that "uch manoeuvres were about the simplest that any army could learn, once it mastered the absolute basics of marching in formation".