Truth is the first casualty in war.

Aeschylus (525-456 BC) Greek dramatist (Æschylus)

(Misattributed)

Variant: "Truth is the first casualty of war."

Not found, as such, in Aeschylus' works. The closest (Fragm. Incert, xi.) is his phrase "God is not averse to deceit in a just cause." Attribution to of the subject phrase to Aeschylus dates only back to 1965. The first recorded use of the phrase as such is from 1915, but even there it is offered as a quotation from an unnamed source.

More discussion of the history of this phrase can be found here and here.