The phrase "Whom the gods would destroy, they first make mad" is a phrase spoken by Prometheus in Henry Wadsworth Longfellow's poem, "The Masque of Pandora" (1875).
Another version ("Those whom the gods wish to destroy they first make mad") is quoted as a "heathen proverb" in Daniel, a Model for Young Men (1854) by William Anderson Scott (1813–1885).
A prior Latin version is "Quos Deus vult perdere, prius dementat" (Life of Samuel Johnson, 1791) but this involves God, (presumably the Christian God) not 'the gods'. An earlier version has Jupiter and can be traced back to the play Antigone by Sophocles. Even this appears to be a borrowing from an earlier, lost Greek play.
This phrase was also used by British politician Enoch Powell in his controversial 1968 speech "Rivers of Blood" for which he was dismissed in disgrace from the Conservative Party Shadow Cabinet, to which he was never to regain any position.