Mettius had an alliance with the Romans while they were rivals of Alba Longa, but in an important battle he withheld his support. It is for this that he is punished; Virgil later recounts this betrayal and its punishment in the Aeneid.
In the battle itself, Mettius, having provoked the inhabitants of Fidenae to attack Rome, retreated to a hilltop with his Alban forces where he waited to see which force would be victorious; he then planned to join the winning side. Tullus Hostilius, the third king of Rome and the king at that time, after winning the battle said that since Mettius was torn between the two cities, the same would be done to his body: his arms were then attached to two chariots that then ran in opposite directions. The result was naturally fatal and remained a warning to all future allies of Rome not to betray her.