Pyrrhic victory 
Pyrrhus, after his victory ofer the Romans, near the river Siris, said to those sent to congratulate him, "One more such victory and Pyrrhus is undone."

From Dictionary of Phrase and Fable: Giving the Derivation, Source, Or Origin of Common Phrases, Allusions and Words that have a Tale to Tell by Ebenezer Cobham Brewer

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