History1918 (World War I)United States / France / Germanyhigh confidence

A memorandum dated June 14, 1918, written in French, reveals the formula the Germans used to produce invisible ink.

Notes on verification

Confirmed by CIA's own official account of its oldest declassified documents, with independent corroboration from Smithsonian Magazine and New York Times coverage of the 2011 declassification. Exact document contents not independently transcribed by third parties, but core facts (date, language, subject) are consistent across sources.

Sources