“Truth," it has been said, "is the first casualty of war.”
Introduction to Truth and the War, by E. D. Morel. London, July 1916. p. ix books.google http://books.google.de/books?id=gQFIAAAAIAAJ&q=casualty. p. xiii in the 3rd edition 1918 archive.org http://www.archive.org/stream/truthwar00more#page/n17/mode/2up (cf. Aeschylus#Misattributed)
Hiram Johnson is often credited with this statement, or something similar. However, Snowden's use appears to have predated those of Johnson while being more consistent with the now-common, "Truth is the first casualty of war."