By 1918 the German army sent most of its tactical traffic by radio, and every word of it was available to the Allies. Field ciphers of the day could not survive that volume: substitution ciphers drowned in frequency tables, transpositions leaked with every repeated phrase. Colonel Max Rongé's office responded in March 1918 with a design by signal officer Fritz Nebel that stacked both defences — a keyed Polybius square (Fractionation) followed by a keyed column shuffle (Transposition).
The first variant used a 5×5 square and the letters A D F G X — chosen because they sound clearly different in Morse code and survive noisy radio links. Within weeks the square gained a sixth row for the digits, and the alphabet grew to A D F G V X. The name followed the alphabet, and German operators entered every message through a strutted "box" of letters and numbers.
Georges Painvin of the French decryption bureau — known for working until his hair turned grey in weeks and he collapsed with tuberculosis — took on the traffic. For months he managed only fragments. The break came from volume: many messages sent each day on the same key let him compare column lengths and match columns across messages, until a long intercepted pair on a Sunday in March 1918 cracked the day's key open.
The decrypted traffic pointed at Amiens. When the German offensive opened there in June 1918, the French were waiting, and the attack stalled. ADFGVX traffic stayed readable for the rest of the war — the Germans kept changing keys daily, the French kept up.
ADFGVX fixed the template that modern ciphers still follow: confuse (substitute) the symbols, then diffuse (transpose) them — in reverse order of what later became Shannon's maxim. Its square also seeded the bifid-style family of fractionating ciphers. Every "two layers are stronger than one" argument in classical cryptography starts here.