Near the close of the sixth century b.c., the Etruscan monarchy ended, but various city-states remained potent forces. One of the chief among them was Veii, on the western bank of the Tiber River just 12 miles upstream from Rome. The two populations were expansionary and were entirely too close to each other to be cooperative. The two were economic rivals as well, for dominance of the Tiber meant control of access to interior markets as well as the salt production from the port city of Ostia, downstream from Rome.

In the 480s a powerful Roman clan, the Fabians, built a fortress-villa roughly halfway between the two cities. The Fabii had familial connections in both cities, but the Fabian strongpoint at the junction of the Tiber and Cremora rivers was too threatening to Veiian security. The battle of Cremora in 476 ended in Veiian control of the key river junction as well as the Janiculum Hill, overlooking Rome itself. This attack broke a truce that the Romans and Etruscan city-states had been observing for nearly forty years. Rome replied to the threat with a siege of Fidenae, a colony also at the junction of the rivers. The siege lasted three years and was ended when the Romans dug under the city walls. Throughout the siege the Veiians had appealed to other Etruscan city-states for aid, but none responded. Veii had broken the truce, they could face the consequences.

The two cities observed an uneasy peace until the end of the fifth century. Then, for reasons that are unclear, Rome began besieging Veii itself. (The dates of the siege are the focus of some dispute. Roman tradition described a ten-year siege from 400 to 390, but most historians think that ten-year period was fabricated to give Rome its own Iliad. Some dates suggested are 405-396, 404-396, or 406-395.)

Appius Claudius was the initial Roman commander. Rome's republican government depended on the landowning citizens to provide military service. Duty and rank depended on income and the ability to arm oneself. As the Romans were primarily an agricultural population, this meant that campaigning was restricted to the summer, after the spring planting and before the fall harvest. Veii was so strongly fortified, however, that it could not possibly be overcome in a few months. Thus, the Roman government reluctantly authorized long-service enlistments with pay and supplies to be provided by the government. This was the beginning of the professional Roman army. A Veiian sally, which destroyed months of labor on the siege works, helped to motivate the government to change the army's recruiting practices.

Three years into the siege, two Etruscan cities launched attacks on Roman camps. They did this in their own interest, thinking Roman armies might target them next. The Veiians, however, thought that all Etruria had finally responded to their call and sent out a sally themselves. The Romans held for a time against this two-pronged assault, but a personal dispute between two Roman commanders ended in forcing a Roman retreat all the way back to their home city. Roman armies returned to the siege the following year and held their positions with little trouble for two more years.

The length of the siege, coupled with a harsh winter, a hot summer, and a plague, began to wear on Roman morale. A solemn banquet was held to honor the gods and invoke their aid, but the nature of Rome's electorate held the key to the problem (or so the patricians thought). Claiming that their problems stemmed from lower-class criticism of the nobility, the tribunal elections brought two patricians to office. One of these tribunes was Marcus Furius Camillus, destined to be Veii's conqueror.

In spite of the seeming placation of the gods, one main omen caused the most anxiety. The Alban Lake, near Rome, rose to an unusual level in the summer of 398. The Romans sent an emissary to Delphi to consult the oracle. In the meantime, a Veiian citizen appeared on the city walls one day and called out in a loud voice that Veii would remain unconquered as long as the Alban Lake remained at its abnormally high level. Under the pretext of requesting a consultation with the wise old Veiian (a haruspex), a Roman soldier kidnapped him and took him to the Senate. There, the haruspex revealed that the gods had turned against Veii; the messenger to Delphi soon returned with an answer that corroborated the old man's prophecy. Thus, work began to dig drainage channels out of the lake's basin.

Rather than bringing about an immediate change in the situation, news from the north alarmed the Romans. The Etruscans were beginning to feel pressure from invading Gauls and had decided to aid Veii, rid themselves of the Roman threat, then put together a united front against the barbarians from the north. This problem motivated the Romans to name the tribune Camillus to the position of dictator. He appointed the able Publius Cornelius Scipio as commander of the cavalry and called for a mass levy of troops. Few failed to respond. The new army won two quick victories over Etruscan troops from Falerii and Capena and gathered immense booty from their camps. Camillus, rather than distribute it to the troops in payment for a job well done, instead sent most of the loot back to Rome, to be used to construct a temple.

With the Etruscan threat beaten back for a time, Camillus returned to Veii and began serious work. He ordered the men to keep their distance from the city walls, and instead to strengthen their trenches. He also began work on a tunnel through the rock supporting Veii's walls. As the tunnel drew nearer its destination, word spread through Rome that the attacking forces would be free to loot Veii at will. That promise motivated a huge percentage of the population to join in the siege. When all was ready, Camillus offered up a prayer to the gods and ordered a massive assault on the walls. The move surprised the defenders, who had been lulled into passivity since Camillus had taken his command and ordered his men to keep away from the walls. As the city leaders gathered in the temple of Juno to ask for direction, a handpicked force of Roman soldiers burst out from their tunnel into the temple and began the slaughter. The troops soon spread through the city, attacking the defenders from the rear and opening the city gates to the horde outside.

Camillus was able to restrain his troops from slaughtering the women and children, who begged for mercy, but the pillage went on unabated. The wealth of the city was greater than anyone had imagined. Fearing the gods might think the mortals too proud of themselves, Camillus asked that a minor misfortune take place in order to show the need for humility. As he finished praying, he turned and stumbled. Picking himself up and dusting himself off, he mused on the rapidity and mildness of the immortals' answer to his plea.

The city of Veii was not only looted of everything of value, it was then completely destroyed. This action was at variance with the normal manner of Roman conquest, which was to absorb the region and population into the Roman social and political realms. Fearing Veii's long-standing power and potential for rebirth, the government deemed a complete destruction in Rome's best interest. From this point forward Etruria put forth little serious opposition to Rome. Unfortunately for the Romans, however, the threat of Gallic invasion proved all too real. In 390 the Gauls defeated a Roman army at the Allia River, then sacked the city. Camillus was reappointed as dictator and managed to drive the Gauls away, but with bribes rather than power. When they returned in 367, however, he once again assumed the position of dictator and this time succeeded in beating back the invaders. No Roman enemy entered the city in victory again for 800 years.