The phrase is named after King Pyrrhus of Epirus, whose army suffered irreplaceable casualties in defeating the Romans at Heraclea in 280 BC and 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 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.
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.
The report is often quoted as "Another such victory over the Romans and we are undone," or "If we are victorious in one more battle with the Romans, we shall be utterly ruined.
While it is most closely associated with a military battle, the term is used by analogy in fields such as business, politics, law, literature, and sport to describe any similar struggle which is ruinous for the victor. For example, the theologian, Reinhold Niebuhr writing of the need for coercion in the cause of justice warned that: "Moral reason must learn how to make a coercion its ally without running the risk of a Pyrrhic victory in which the ally exploits and negates the triumph"