From 1942, Hermes was in touch with members of the resistance like Carl Friedrich Goerdeler and the Kreisau Circle. Immediately after 20 July 1944 he was arrested and on 11 January 1945 sentenced to death for being an active member of the conspiracy to kill Hitler. In one of Goerdeler's lists he was named as a post-putsch minister for agriculture. However, his wife succeeded in delaying his execution until the Red Army occupied Berlin.