A student of law, Gessler became mayor of Regensburg (1910–11) and Nürnberg (1913–19). After the German revolution of November 1918, he helped found the bourgeois-liberal German Democratic Party (DDP) and served as minister of reconstruction (1919–20). In March 1920 he was appointed minister of war, a position he held through changing coalition governments for the next eight years. In the autumn of 1923 he was briefly granted supreme power to restore order against extremist elements of both the left and the right. As war minister, he allowed the Reichswehr (army) to increase its size beyond the provisions of the Versailles Treaty. Increasingly unpopular with the left opposition in parliament, he was finally forced to resign in January 1928 over a financial scandal involving the army. During World War II he was arrested for complicity in the July 20, 1944, attempt on Adolf Hitler's life and spent seven months in a concentration camp. After the war he served until his death as head of the Bavarian and German Red Cross organizations.