He saw action at first hand in the Russian-Polish War of 1919 - 1920, fighting alongside the Poles. Despite capture by the Soviet Russian forces, he managed to escape and thus avoid execution.
After the fall of France and in the wake of the chaotic evacuations at Dunkirk in 1940, Yeo-Thomas escaped back to England.
He worked initially as an interpreter for de Gaulle's Free French forces. His talents were quickly prised away from de Gaulle by SOE, who used him as a liaison officer between SOE and BCRA[?], the Free French intelligence agency.
He quickly forged links with Major Pierre Brosselet[?] and Andre Dewavrin[?] (who went under the pseudonymous codename of Colonel Passy[?]), and between them they set in train a strategy for thwarting and obstructing the German occupation of France.
He surreptitiously visited France on a number of occasions. He was appalled at the paucity of logistical and material support which the French resistance movemements such as the maquis were receiving, to the extent that he begged five minutes with Winston Churchill, the British Prime Minister. Churchill reluctantly agreed, but was fascinated by what Yeo-Thomas told him and agreed to help him obtain resources, which were forthcoming.
In February 1944 Yeo-Thomas was parachuted into France, but was betrayed and captured at the Passy metro station in Paris. He was taken by the Gestapo to the HQ in Avenue Foch, and was subjected to brutal torture as a part of his interrogation. He made two failed attempts to escape and was eventually transferred to Compeigne prison[?] and from there to Buchenwald concentration camp[?]. Within the camp, he began to organise resistance, and again made an escape attempt. On his recapture, he passed himself off as a French national and was returned to a camp near Marienburg.
After the war Yeo-Thomas was to be an important witness at the Nuremberg war trials[?] in the identification of Buchenwald officials.