In 282 BC, after a battle against the Samnites, Lucanians, Bruttians and Thurii, Roman troops entered the Italian Greek colonies of Croton, Lokroi, and Rhegium.

According to Kęciek, the Tarentine aristocracy asked the Roman commanders Publius Cornelius and Lucius Valerius to arrest and execute the democrats and their followers, which would allow the aristocrats to surrender.

The Romans sent a diplomatic mission to settle the matter and take back the prisoners but the negotiations ended abruptly, so Rome declared war on Tarentum.

He also believed that he could count on help from the Samnites, Lucanians, Etruscans, Umbrians, and Bruttians, and some Illyrian tribes, all peoples with a history of conflict with Rome.

Before he left Epirus, Pyrrhus formed an alliance and borrowed soldiers and money from the pretender to the Macedonian throne, Ptolemy Keraunos.

His long-time friend and ally Ptolemy II Philadelphus, king of Ptolemaic Egypt, also promised to send 9,000 soldiers and 50 war elephants.

They divided it into four armies: Publius Laevinus moved towards Heraclea, a city founded by the Tarentines, with the intention of cutting Pyrrhus off from the Greek colonies of Calabria, thus avoiding their uprising against Rome.

When he understood that reinforcements were not coming, he decided to fight the Romans on a plain near the river Siris (modern Sinni), between Pandosia and Heraclea.

Before the fight he sent diplomats to the Roman consul, proposing that he could arbitrate the conflicts between Rome and the population of southern Italy.

He took up position on the left bank of the Siris, hoping that the Romans would have difficulty crossing the river, which would allow him more time to prepare his attack.

He set up some light infantry units near the river to let him know when the Romans began to cross, and planned first to attack them with his cavalry and elephants.

The infantry line was near equal to the Romans' in length as, although Pyrrhus had a small advantage in number, the phalanx was by design deeper than the legion.

Grasping the magnitude of the situation, Pyrrhus rode forward, bare-headed, along the lines of his men to show he was still living.

Aghast at the sight of these strange and brooding creatures which none had seen before, the horses galloped away and threw the Roman legion into rout.

The Greeks of Rhegium who wanted to join him were massacred by Roman soldiers under the command of Decius Vibelius, who was proclaimed ruler of the town.