“Truth is not the first casualty of war alone: it is the first casualty of populism.”

Source: Executive Power

Widely attributed to Johnson, but without any confirmed citations of original source: "The first casualty when war comes is truth," remarked Hiram Johnson, "and whenever an individual nation seeks to coerce by force of arms another, it always acts, and insists that it acts in self-defense" (Locomotive Engineers Journal, February 1929, p. 109). Arthur Ponsonby earlier said: "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 expressed a similar idea: "Among the calamities of war may be jointly numbered the diminution of the love of truth, by the falsehoods which interest dictates and credulity encourages." Cf. Aeschylus#Misattributed.
Attributed

http://books.google.de/books?id=T2OsAAAAMAAJ&q=%22declared,+truth+is+the+first%22&dq=%22declared,+truth+is+the+first%22&hl=de&sa=X&ei=NNNeT5XTNoSi4gTpuOnRBw&ved=0CDwQ6AEwAQ books.google
This famous quotation is similar to the one Philip Snowden used 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." Cf. Aeschylus#Misattributed.
Falsehood in Wartime (1928)

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: The General in His Labyrinth

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

Source: Life, the Truth, and Being Free (2010), p. 52

"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.