What is the origin of the idiom "Pyrrhic victory" and when to use it?

The phrase "Pyrrhic victory" is named after King Pyrrhus of Epirus, who won a battle against the Romans in 279 BC, but suffered such heavy losses that he was unable to capitalize on his victory and ultimately lost the war. In modern usage, this idiom is often used to describe a situation where a victory is achieved, but at a cost that outweighs the benefits.