- Never near Argos -

[Mediolanicus]

Book III, 1-3.

Background:

Pyrrhos’ losses in the Battle of Cannae were limited. He had lost three of his prized elephants and though his Italian allies had suffered some more casualties, the core of his army remained in top strength. The victory over the Romans was clear, unlike the his last victory over this opponent.
It is my opinion that Pyrrhos now made a critical error. He had just beaten the main Roman force. The other legions were busy fighting over Rhegion and defending Ariminum from Celtic raids. The Roman tactic to counter his phalanxes had miserably failed and thanks to his Italian allies he had a well balanced and motivated army. The way to Arpi lay open, but Pyrrhos did not move.

The text:

[…]
The leader of the Samnites, a man called Paius, urged the Epirote king to move North and take Safinim [this is Samnium in the Oscan dialect]. Paius told the general he could invade Apulia, take Arpi and then push [west] towards Campania.
[…]
On hearing that the army was not going to do more than patrolling the border of the Tarantine territory, Paius stepped into the general’s tent, holding high a dagger on both his hand palms.
The guards worriedly pulled their swords, but the king stopped them and invited the Samnite to come closer. The man stopped, kneeled, put the dagger on the ground in front of Pyrrhos and left the tent without saying a word.
[…]
Pyrrhos’ forces thus lost between one forth and one third of their strength.
[…]

Notes:

Staying on the defensive had cost Pyrrhos his Samnites, who had fought so bravely in the Battle of Cannae. They did not want to fight for the cause of a Greek city, they wanted to be freed from Roman oppression. In Pyrrhos they saw a potential victor over the Romans, someone who could give them their homelands back.
In my view he should have marched upon Arpi right after the battle, taking the city undefended and demoralized after the news of the Roman loss. Pyrrhos failed to do so, perhaps because he preferred field battles over sieges, or perhaps because he was afraid the army in Rhegion would hit him in the back. We all know that this army could not have given up the siege of Rhegion. And the casualties it suffered during the actual taking of Rhegion were so high it made the army incapable to fight Pyrrhos anytime soon.
My view is that the decision not to attack Arpi, making him lose up to one third of his army through desertion and giving the Romans time to heal their wounds, was his greatest mistake as a general. And though my readers are of course free to think otherwise, I have yet to meet someone who does not agree with my view on this.