[Xietor]

In response to a PM:

A Pyrrhic victory is a victory with devastating cost to the victor. The phrase is an allusion to King Pyrrhus of Epirus, whose army suffered irreplaceable casualties when he defeated the Romans during the Pyrrhic War at Heraclea in 280 BC and Asculum in 279 BC. After the latter battle, Plutarch relates in a report by Dionysius:
“ The armies separated; and, it is said, Pyrrhus replied to one that gave him joy of his victory that one more such victory would utterly undo him. For he had lost a great part of the forces he brought with him, and almost all his particular friends and principal commanders; there were no others there to make recruits, and he found the confederates in Italy backward. On the other hand, as from a fountain continually flowing out of the city, the Roman camp was quickly and plentifully filled up with fresh men, not at all abating in courage for the loss they sustained, but even from their very anger gaining new force and resolution to go on with the war. [1] ”

In both of Pyrrhus's victories, the Romans lost more men than Pyrrhus did. However, the Romans had a much larger supply of men from which to draw soldiers, so their losses did less damage to their war effort than Pyrrhus's losses did to his.