 "- Jan Mosdorf "Wczoraj i Jutro", 1938

In late 1939, after the Polish September Campaign, he returned to the underground National Party.He was one of leaders of the party, and he helped with the creation of the anti-Nazi paramilitary units known as Narodowa Organizacja Wojskowa.In July 1940, Mosdorf was arrested and placed in Gestapo's infamous Pawiak prison.