Pyrrhic victory
A Pyrrhic victory is a victory which is only achieved with heavy losses on one's own side.
Pyrrhus was the King of Epirus from 295-272 BC. He entered Italy with 20,000 foot soldiers, 3,000 cavalry men, 2,000 archers, 500 hurlers and 20 elephants to help Tarentum with their struggle against the Romans.
Due to his superior cavalry and his elephants he won over the Romans at Heraclea under their consul Publius Valerius Laevinus in 280 BC. He then offered a peace treaty, which was rejected by the Romans. Later (279 BC), the battle at Ausculum took place. This is the one the phrase refers to, because Pyrrhus is recorded to have commented it by saying: "If we win another battle against the Romans, we will be completely lost" (Plutarch, Pyrrhus 21,14).
Pyrrhus' mercenary soldiers were more expensive, but also better fighters than the Roman conscripts. They won every fight against the Romans, but the cost of replacing killed mercenaries battle for battle proved too heavy a financial burden.