The army of the Veientines had crossed the Tiber at a spot between his camp and Fidenae. In the battle they formed the right wing near the river, the Fidenates were on the left nearer the mountains. Tullus formed his troops in front of the Veientines, and stationed the Albans against the legion of the Fidenates. The Alban general showed as little courage as fidelity; afraid either to keep his ground or to openly desert, he drew away gradually towards the mountains.

When he thought he had retired far enough, he halted his entire army, and still irresolute, he began to form his men for attack, by way of gaining time, intending to throw his strength on the winning side.

Those Romans who had been stationed next to the Albans were astounded to find that their allies had withdrawn and left their flank exposed, when a horseman rode up at full speed and reported to the king that the Albans were leaving the field. In this critical situation, Tullus vowed to found a college of twelve Salii and to build temples to Pallor and Pavor.

Then, reprimanding the horseman loud enough for the enemy to hear, he ordered him to rejoin the fighting line, adding that there was no occasion for alarm, as it was by his orders that the Alban army was making a circuit that they might fall on the unprotected rear of the Fidenates.

At the same time he ordered the cavalry to raise their spears; this action hid the retreating Alban army from a large part of the Roman infantry. Those who had seen them, thinking that what the king had said was actually the case, fought all the more keenly. It was now the enemies' turn to be alarmed; they had heard clearly the words of the king, and, moreover, a large part of the Fidenates who had formerly joined the Roman colonists understood Latin.

Fearing to be cut off from their town by a sudden charge of the Albans from the hills, they retreated. Tullus pressed the attack, and after routing the Fidenates, returned to attack the Veientines with greater confidence, as they were already demoralized by the panic of their allies.