It is unknown how many troops Pyrrhus had left in Tarentum, it is estimated he had about 25–35,000 troops with him at Heraclea. 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. Valerius Laevinus had about 42,000 soldiers under his command, including cavalry, velites, and spearmen. It would be the first time in history that two very different juggernauts of war clashed: the Roman Legion and the Macedonian Phalanx.