“A Pyrrhic victory is a victory that inflicts such a devastating toll on the victor that it is tantamount to defeat. Someone who wins a Pyrrhic victory has been victorious in some way, though the heavy toll negates a true sense of achievement or profit.
Pyrrhic victory is named after king Pyrrhus of Epirus, whose army suffered irreplaceable casualties in defeating the Romans at the Battle of Heraclea in 280 BC and the Battle of Asculum in 279 BC, during the Pyrrhic War. 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 other 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.
— Plutarch, Life of Pyrrhus
In both Epirote victories, the Romans suffered greater casualties but they had a much larger pool of replacements, so the casualties had less impact on the Roman war effort than the losses of King Pyrrhus.
The report is often quoted as
Ne ego si iterum eodem modo uicero, sine ullo milite Epirum reuertar.
Another such victory and I come back to Epirus alone.