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; however, the heavy toll negates any sense of achievement or profit (another term for this would be "hollow victory").
The phrase Pyrrhic victory 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 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 
In both of Pyrrhus's victories, the Romans suffered greater casualties than Pyrrhus did. However, the Romans had a much larger supply of men from which to draw soldiers and their casualties did less damage to their war effort than Pyrrhus's casualties did to his.
The term is used as an analogy in fields such as business, politics, and sports to describe struggles that end up ruining the victor. Theologian Reinhold Niebuhr, for example, commented on the necessity of coercion in preserving the course of justice by warning, "Moral reason must learn how to make coercion its ally without running the risk of a Pyrrhic victory in which the ally exploits and negates the triumph." Also, in Beauharnais v. Illinois, a 1952 U.S. Supreme Court decision involving a charge proscribing group libel, Justice Hugo Black alluded to the Pyrrhic War in his dissent: "If minority groups hail this holding as their victory, they might consider the possible relevancy of this ancient remark: 'Another such victory and I am undone.'"
A related expression is "winning a battle but losing the war", describing a poor strategy that wins a lesser (or sub-) objective but overlooks and loses the truly intended objective. This contrasts with a Pyrrhic victory in which the objective is achieved but at a cost that makes the victory "turn to ashes".
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- Pinker, Steven (2011). The Better Angels Of Our Nature. Pg.263: Penguin. ISBN 978-0-141-03464-5.
- Plutarch (trans. John Dryden) Pyrrhus, hosted on The Internet Classics Archive
- "Ne ego si iterum eodem modo uicero, sine ullo milite Epirum reuertar": Orosius, Historiarum Adversum Paganos Libri, IV, 1.15.
- Plutarch, Life of Pyrrhus, 21:8.
- Niebuhr, Reinhold Moral Man and Immoral Society, published by Scribner, 1932 and 1960, reprinted by Westminster John Knox Press, 2002, ISBN 0-664-22474-1, ISBN 978-0-664-22474-5 p. 238.
- Beauharnais v. Illinois., 343 250 (U.S. 1952).
- Denson, John, The Costs of War: America's Pyrrhic Victories. Transaction Publishers (1997). ISBN 1-56000-319-7.