He was a member of the resistance during World War II and afterwards one of the founders of the Christian Social Union (CSU).

The correspondence and related intelligence passed through an intermediary to the hands of Pope Pius XII, who would review it and in turn forward it to Lord Halifax in Britain.

Dohnanyi summarized the material into a report, containing list of individuals slated to assume roles in a post-coup civilian government.

The Pope's Private Secretary, Robert Leiber, acted as the intermediary for Pius and met with Müller, who visited Rome in 1939 and 1940.

[2] The Vatican considered Müller to be a representative of Colonel-General Beck and agreed to offer the machinery for mediation between the German Resistance and the Allies.

[3] The Vatican agreed to send a letter outlining the basis for peace with England and the participation of the Pope was used to try to persuade senior German Generals Halder and Brauchitsch to act against Hitler.

Unlike fellow inmates Canaris, Oster and Bonhoeffer, who were executed in April 1945, Müller was spared at the last moment, at the foot of the scaffold, through the intervention of Johann Rattenhuber who convinced Ernst Kaltenbrunner that keeping Müller alive might help Germany negotiate more favourable surrender terms.

[8] Instead, Müller was transferred to Tyrol in late April 1945 along with 138 other "special prisoners" (Sonderhäftlinge) and "kin prisoners" (Sippenhäftlinge), persons of prominence the Nazi SS had hauled off in the final days of the war to Niederdorf, South Tyrol, where they were to be hidden and used as bargaining chips.