In 2005, the work of Hilmar-Detlef Brückner of the Bavarian State Archive (German: Bayerisches Hauptstaatsarchiv) brought this aspect of Föppl's career to prominence.

His brother was Otto Föppl who was an engineer and Professor of Applied Mechanics at the Technical University of Braunschweig for 30 years.

[1][5] He worked as an assistant to Felix Klein, a leading mathematician on Group theory, complex analysis and non-Euclidean geometry.

The unit's remit was to intercept wireless communications of the British Expeditionary Force and the ships of the Royal Navy in the English Channel.

However, Föppl ended up working as a kitchen boy, as he was unable to transcribe nonencrypted messages due to his lack of French.

Föppl suggested it was used as it enabled the cipher clerk to encrypt messages very quickly and could be easily enciphered by ordinary sailors, with a key that was changed every few weeks, that could be broken within a single day.

Föppl found that due to his work, he was able to rescue academically gifted individuals from the front line, for use as cryptanalysts and evaluators, an idealised sentiment which was not always successfully achieved.

By the end of World War I, Föppl was head of Sixth Army’s Evaluation Office, located in Lille and then Tournai.

In March 1938, Föppl was reactivated and told to report to Army HQ after the Anschluss, where he was sent to Vienna to work at a German wireless company.

Upon arrival, there was some confusion as to his purpose, since he was then in his mid 50s, with no modern uniform, and had spent the interwar period employed as an academic.

[1] On 25 August 1939, Föppl was again reactivated and requested (ordered) to Army HQ in Berlin for assignment, which put his whole family into a deep depression.

He subsequently made a request to move back to work at the Technical University of Munich and was discharged on 20 January 1940, with his military career at an end.

During the Second World War, he relocated his residence and photoelasticity laboratory to Ammerland, which may have saved his life, because his house in Munich was hit by an American bomb in an attack on July 12, 1944, and was completely destroyed.