The battle of Lake Regillus (499 or 496 BC) was a narrow Roman victory over the Latin League early in the life of the Republic that helped to prevent the last of the kings of Rome from regaining his throne.
Rome under the kings may have dominated Latium, but the expulsion of the last of the kings and the campaign of Lars Porsenna allowed the cities of the Latin League to escape Roman control (if it had ever existed).
Livy is our main source for the events of the war between Rome and the Latin League, and even he admitted that the dates of the events couldn't be accurately pinned down. In Livy's account the war broke out during the consulships of T. Aebutius and C. Vetusius, but the Roman army at Lake Regullus was commanded by the Dictator Aulus Postumius and his master of horse T. Aebutius. By Livy's time two different historical traditions existed, one dating the war to this first year and another placing it three years later. In later Roman history the appearance of a Dictator would indicate that there had been some sort of disaster, but in the 490s the Roman Republic was in its infancy, having only been proclaimed in 509 B.C., and the constitution was still evolving so that may not have been the case.
The Latin army was commanded by Octavius Mamilius of Tusculum, the son-in-law of the last Roman king. As well as the Latin troops it also contained a group of Roman exiles led by the young Lucius Tarquinius, while L. Tarquinius Superbus, the deposed last king of Rome was present with the army.
The battle was fought at Lake Regillus, close to Tusculum (to the south-east of Rome). According to Livy, who gives a detailed account of the battle, most of the senior officers on both sides were wounded. The deposed king saw the Roman dictator addressing his men and charged him on horseback. Roman troops intervened, wounding Tarquinius Superbus, who was taken to safety. On the other flank of the battle Aebutius clashed with Octavius Mamilius, and in the clash both men were wounded and forced to retreat. The Latins were now under severe pressure, and so Mamilius called in Lucius Tarquinius and the Roman exiles, who temporarily forced the Romans to retreat. An attempt by M. Valerius to restore the situation ended with his death, and the Romans were close to defeat.
Postumius responded by ordered his own bodyguard to attack any Roman troops who attempted to retreat. This restored the line, and when the Dictator's bodyguard joined the fight the Roman exiles were almost surrounded. Mamilius responded by leading his reserves into the battle, but was killed in single combat with T. Herminius, who later died of his wounds.
Despite the death of their leader the Latins were still holding out, and so Postumius ordered the Roman cavalry to dismount and fight on foot. He also vowed to build a temple to Castor and Pollox, the cavalry gods if the battle was won (the temple was dedicated in 484 B.C.). According to a later Greek myth the gods themselves, mounted on white horses, helped the Romans to victory.
According to Livy the appearance of the Roman noble cavalry in the front ranks decided the battle. As the Latins began to retreat the cavalry remounted and charged, finally breaking the Latin line and also capturing their camp.
The battle of Lake Regillus didn't end the war, but it does seem to have ended the fighting - Livy records three years of 'neither settled peace nor open war'. The war finally ended with the signing of a treaty between Rome and the Latin League, traditionally dated to 493 B.C. (after the death of Tarquinius Superbus in exile). A summary of the terms of the treaty can be found in the works of Dionysius of Halicarnassus, written at a time when the original bronze pillar with the treaty text was still on display in Rome.
This was a treaty between equals. Rome and the Latin League agreed to form a defensive military alliance, to perpetual peace between the two parties, not to assist or give free passage to the enemies of the other and to split the spoils of any successful campaign equally. This alliance was probably forced on the two sides by the invasions of the Volsci and the Aequi, two Italic peoples who threatened to overrun Latium. It remained the basis of relationships between Rome and the Latins for the next century and a half, until the end of the Latin War.