Pyrrhic Victory

A Pyrrhic Victory is a victory that causes devastating loss to the victor. This term is named after King Pyrrhus of Epirus, who suffered many casualties after defeating the Romans at Heraclea in 280 BC and Asculum in 279 BC during the Pyrrhic wars between the Greeks and Romans. In every battle fought with the Romans, the Roman's lost more men but in turn they also had a larger supply of men to draw from. Therefore Pyhrrus lost more men in the long run, and eventually lost the war in the long run. The campaign is described by Dionysius in the following writing:
"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."
Some modern examples would be the Battle of Bunker (Breed's) Hill during the Revolution in which the Americans killed and wounded hundreds of British soldiers while only losing a few of their own by comparison. Despite all the casualties that the British took, they still took the hill. Another example of a Pyrrhic victory is Napoleon's siege of Moscow. Though he took the city of Moscow, he lost over 1/3 of his army in the venture. This is the ultimate form of a Pyrrhic Victory.

http://en.wikipedia.org/wiki/Phyrric_victory