the monarchy that rome had cast out refused to die in silence. its last attempt to return provoked a battle so desperate that, in roman memory, the real history ended up fusing with myth.
the ancient sources do not agree on the date: livy recounts the battle under the dictatorship of aulus postumius albus — which varronian chronology dates to around 499 bce — though he himself admits that his sources disagreed; dionysius of halicarnassus, by contrast, places it in 496, and that is the year tradition came to favour and the date usually cited, though it is worth remembering the chronicles themselves disagreed. what none disputes is what was at stake. a powerful coalition of latin cities marched against rome with an enormous army; in the field it was led by octavius mamilius, dictator of tusculum and tarquin’s son-in-law, while the deposed king backed it from exile. the young republic was staking its existence on a single day.
with the roman line on the point of giving way from exhaustion, the commander promised the gods a temple in exchange for victory.
the two armies clashed on the shores of lake regillus, in latin territory, in a hand-to-hand slaughter between phalanxes. it was a fight of shoving and attrition, with no brilliant manoeuvres: two shield-walls pressing until one gave way. and the roman line, exhausted, was about to crack. it was then, the story goes, that the dictator in command, aulus postumius, made his vow to castor and pollux, the dioscuri, the divine twins, promising them a temple in the heart of rome in exchange for victory.
legend completed the scene: two colossal horsemen on white horses materialised at the head of the roman formation, swept the enemy line aside and decided the battle. that very evening, tradition has it, two strangers of superhuman stature were seen watering their horses at a fountain in the forum, announcing the victory before any messenger arrived. rome kept the vow: the temple of the castors rose in the forum and the battle was sealed as an episode in which the divine had intervened in person. historians read that prodigy for what it is, a legendary construction that dignified the victory and gave religious sanction to the new regime; but the military outcome — the defeat of the attempted restoration — is taken as a plausible historical kernel.
with regillus, the threat from the kings was finished off. tarquin died shortly afterwards in exile, without throne or army, and rome was at last free of kings. the episode closed the cycle that had been opened by the expulsion of the monarchy and consolidated the young republic against its neighbours.
its most lasting consequence, however, was not military but diplomatic. a few years later, the tradition says, rome and the latin cities concluded an alliance — the so-called treaty of cassius — that ended the war and bound them as equal partners to share spoils and to defend themselves against common enemies. the latins thus moved from defeated rivals to allies, and rome from threatened city to head of a coalition. that turn laid the foundations of all its future expansion: the roman method of integrating and allying with the defeated, rather than merely crushing them, was born in large measure from the world regillus left behind. the battle legend won with gods on horseback opened, in practice, rome’s long road to dominion over latium.
but the victory carried a bitter social irony. the plebeian infantry that had provided most of the dead at regillus came back to the capital to find that the patrician elite was ruining them with implacable debt laws. the same men who had just saved rome from the kings discovered that, behind closed doors, they were still potential serfs. they had defended the liberty of the republic without sharing in it. out of that indignation — that of the soldier who bleeds for a city that treats him as a debtor — would soon emerge the first great organised protest in the history of the west.
