During his studies in Heidelberg, Liepmann founded the Akademischdemokratische Studentengruppe (Democratic Student Group) which was closely allied with the DDP, and led it from 1926 till 1929.
He demonstrated, using statistical material, that tariffs for all product categories, and in particular for agricultural produce, had risen sharply in the five years following the failed World Economic Conference of 1927, and he was a critic of British protectionism.
In early 1940 the Reich Security Main Office added him to The Black Book, an index of persons who, in the event of a successful invasion of Britain by the German army, were to be found and arrested as a matter of high priority by SS Commandos who would follow the occupying forces.
[1] In early 1940 Liepmann was offered research funding at Queen's University in Kingston, Ontario, but was unable to take it up owing to his internment by the British authorities in May 1940 as an enemy alien.
In 1942 he received a scholarship to work at the Royal Institute of International Affairs/Chatham House in Cambridge; then from April to September 1943 he was employed for his linguistic skills as a monitor by the BBC.