“The first casualty when war comes is truth.”

It is the inescapable duty of every decent citizen to express no interest in or enthusiasm for football and the World Cup http://www.socialaffairsunit.org.uk/blog/archives/000966.php (June 7, 2006).
The Social Affairs Unit (2006 - 2008)

This is often attributed to U.S. Senator Hiram Johnson, but does not appear anywhere in his speeches. Arthur Ponsonby#Falsehood in Wartime (1928) quoted: "When war is declared, Truth is the first casualty", but the first recorded use seems to be by Philip Snowden in his introduction to Truth and the War, by E. D. Morel. London, July 1916: "'Truth,' it has been said, 'is the first casualty of war.'" Samuel Johnson#The Idler (1758–1760) expressed a similar idea: "Among the calamities of war may be justly numbered the diminution of the love of truth, by the falsehoods which interest dictates and credulity encourages."
Misattributed

Source: Executive Power

Source: The Ramal Extraction (2012), Chapter 12

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."

Source: Money: Whence It Came, Where It Went (1975), Chapter VIII, The Great Compromise, p. 92

Source: The General in His Labyrinth

"Empire of Lies" Presented to the Libertarian Party of New Mexico, Albuquerque, New Mexico, 15 June 2003 http://www.ncc-1776.org/tle2003/libe228-20030622-01.html.

Must We Go to War? (1937)