Pyrrhus had not yet been joined by his allies and took to the field with his forces. He set up his camp on the plain between the cities of Pandosia and Heracleia. He then went to see the Roman camp further along the River Siris. He decided to delay to wait for his allies and, hoping that the supplies of the Romans, who were in hostile territory, would fail, placed guards by the river. The Romans decided to move before his allies would arrive and forded the river. The guards withdrew. Pyrrhus, now worried, placed the infantry in battle line and advanced with the cavalry, hoping to catch the Romans while they were still crossing. Seeing the large Roman infantry and cavalry advancing towards him, Pyrrhus formed a close formation and attacked. The Roman cavalry began to give way and Pyrrhus called in his infantry. The battle remained undecided for a long time. The Romans were pushed back by the elephants and their horses were frightened of them. Pyrrhus then deployed the Thessalian cavalry. The Romans were thrown into confusion and were routed.