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                                        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."
                                    
 
        
     
                             
                             
                             
                             
                            