1918: "Mohawk" Field Code was issued.
Trench codes were codes used for secrecy by field armies in World War I. A reasonably-designed code is generally more difficult to crack than a classical cipher, but of course suffers from the difficulty of preparing, distributing, and protecting codebooks. The Americans adopted a series of codes named after rivers, beginning with "Potomac". They learned to print the codebooks on paper that burned easily and degraded quickly after a few weeks, when the codes would presumably be obsolete, while using a font that was easy to read under trench conditions.