The battle of the Anio (437 or 428 B.C.) was a Roman victory early in the Second Veientine War that was won after Lars Tolumnius, king of Veii, was killed in single combat.
The war broken out after the city of Fidenae, five miles upstream of Rome, rebelled against Roman authority, murdered four ambassadors, and successfully sought aid from Veii. The combined Veientine and Fidenate army had crossed the River Anio, directly threatening Roman territory, but had then been defeated by the consul L. Sergius Fidenas and retreated back across the Anio. Lars Tolumnius had then taken up a position in the hills between the Anio and Fidenae, where he waited for the arrival of an allied contingent from Falerii.
This early Roman victory had been won at such a heavy cost that the Senate decided to appoint Mamercus Aemilius as Dictator. L. Quinctius Cincinnatus became his Master of Horse, with Quinctius Capitolinus and M. Fabius Vibulanus served as senior officers (described as his two seconds in command by Livy). Aemilius then crossed the Anio and took up a position in the angle between that river and the Tiber. On the following day he advanced towards Tolumnius's position and offered battle.
Tolumnius had something of a dilemma. His own men, and the Fidenates, wanted to hold their ground and force the Romans to attack. In contrast the Falerii, who had now arrived, were eager to fight a battle. According to Livy the Allied army outnumbered the Romans, and so when the Romans offered battle Tolumnius decided that he too would fight, but not until the following day.
On the day of the battle both armies emerged from their camps and formed up on a plain between the two camps. Tolumnius placed his own troops on the right, the Faliscans on the left and the Fidenates in the centre, and he also send a detachment around the hills in an attempt to surprise the Roman camp. The two armies then stood facing each other for some time before the battle began. It is a sign of how just how close to Rome the battlefield was that Mamercus Aemilius was said to be watching the citadel of Rome waiting for a signal from the augurs. Tolumnius was presumably waiting for his detachment to reach the Roman camp before launching his attack.
The Roman augurs produced good omens first. In Livy's account of the battle the Roman cavalry charged the Etruscan lines and forced their infantry to retreat. Lars Tolumnius, at the head of the Etruscan cavalry, took advantage of the resulting disorder in the Roman cavalry and restored the situation.
The battle was decided by the actions of A. Cornelius Cossus, a military tribune in the Roman cavalry. During what would appear to have been a rather confused cavalry battle Cossus caught sight of Tolumnius, charged him and knocked the Veientine king off his horse. Before Tolumnius could get back to his feet Cossus dismounted and killed him with repeated spear thrusts. This made him the first historical figure to kill an enemy king in single combat (Romulus was the only other figure in Roman history to have done this), winning the spolia opima - the defeated king's armour (and possibly his head). These spoils were later dedicated to Jupiter Feretrius.
The death of their king ended Etruscan resistance. Livy records a great slaughter, but does admit that the Fidenates managed to escape into the hills.
The detachment sent against the Roman camp did eventually reach its objective. While the Etruscans were attempting to break into the camp at one point, Fabius Vibulanus made a sortie from one of the other gates (the Porta Principalis), attacking the Etruscans in the flank and forcing them to retreat.
After the battle Cossus crossed the Tiber and lead a cavalry raid across Veientine territory before returning to Rome to take part in the Dictator's triumph. This victory ended any direct threat to Rome, but the war itself dragged on for several more years, partly because of an epidemic in Rome. Only after the eventual siege and fall of Fidenae did the war finally come to an end.