He became part of a Berlin-based anti-fascist resistance group during World War II, that was later named the Red Orchestra ("Rote Kapelle") by the Abwehr.

[1] Thiel along with his wife Hannelore were most notable for printing stickers using a child's toy rubber stamp kit, that they used to protest The Soviet Paradise exhibition (German original title "Das Sowjet-Paradies") in May 1942 in Berlin,[2] that was held by the German regime to justify the war with the Soviet Union.

As he was part of the communist youth group, he was detained by the Gestapo in 1936, in suspicion of High Treason but was released for lack of evidence.

At the Abendgymnasium, he made friends with his fellow students Friedrich Rehmer, Otto Gollnow, Ursula Goetze and Eva Rittmeister.

Thiel participated in the production and distribution of the programmatic pamphlet Die Sorge um Deutschlands Zukunft geht durch das Volk,[10] supported Hans Coppi in his attempts to repair defective radios and to learn radio, received and passed on leaflets and writings.

In detention, Thiel was severely tortured psychologically and physically (among other things, he was exposed to ultraviolet radiation for days).

On 18 January 1943, Thiel was sentenced to death for "preparation of high treason" and "for favouring the enemy" by the 2nd Senate of the Reichskriegsgericht.