As Adolf Hitler came to power in Germany, Hermann participated in the underground movement against the Nazis, but by 1936 she left Germany for Denmark and later England. She returned when World War II was over. In her later years she was more interested in politics and philosophy than in physics and mathematics.
As a philosopher, Hermann had a particular interest in the foundations of physics. In 1935 she discovered a flaw in John von Neumann's purported 1932 proof that a hidden variable theory of quantum mechanics was impossible. The result went unnoticed by the physics community until it was rediscovered by John Stewart Bell.