Following his release, he became a Nazi activist and lawyer whose high-profile cases included the defence of neo-Nazis and genocide deniers while simultaneously promoting denial and the movement's organisations.

[3] From 1936 until 1937, Herrmann served in Condor Legion during the Spanish Civil War, travelling on the Usaramo to Cadiz in early August 1936.

One of his initial tasks in Spain included developing instructions and training for the usage of the 2 cm Flak 30 anti-aircraft artillery.

[8] World War II in Europe began on Friday 1 September 1939 when German forces invaded Poland.

Following the 1939 aerial Battle of the Heligoland Bight, Royal Air Force (RAF) attacks shifted to the cover of darkness, initiating the Defence of the Reich campaign.

Each sector named a Himmelbett (canopy bed) would direct the night fighter into visual range with target bombers.

By March 1943, Herrmann had managed to secure a Focke-Wulf Fw 190 fighter at Berlin-Staaken airfield and flew several training missions in mock combat against a Heinkel He 111 bomber.

In April, Herrmann put his ideas to the test when Himmelbett detection reported a De Havilland Mosquito bomber heading for Berlin.

On 5 September, Herrmann was injured in a flight accident when his Fieseler Fi 156 Storch C-3 (Werknummer 1297—factory number) suffered engine failure during takeoff at Hennef.

[18] On 12 October, Herrmann was again injured when technical malfunction of his Messerschmitt Me 210 A-1 (Werknummer 6019) forced him to make a gear-up landing at Le Bourget Airfield.

Herrmann was a leading exponent of the tactical deployment of Rammjäger Sonderkommando Elbe (ram fighters), sent into action in April 1945.

Herrmann's intention was to gather a large number of these fighters for a one-off attack on the USAAF bomber formations in the hope of causing enough losses to curtail the bombing offensive for a few months.

Fuel shortages prevented employment of the large numbers necessary, although from one mission of this type, on 7 April 1945, of the 120 fighters thus committed only 15 returned.

He defended Otto Ernst Remer, the head of the neo-Nazi Socialist Reich Party and the Holocaust deniers David Irving and Fred A.

[citation needed] Hermann's affinity for Remer, a committed Nazi and former Wehrmacht officer, was rooted in their mutual Holocaust denial activities.