On 10 October 1806 a united Prussian and Saxon contingent met with a corps of the French Grande Armée under Marshall Jean Lannes at the Battle of Saalfeld, whereby Prince Louis Ferdinand of Prussia was killed. The opening of the Leipzig-Probstzella railway further boosted the town's development. After World War I and the German Revolution of 1918–1919, Saalfeld became part of the newly established Free State of Thuringia. As a railway junction and garrison town of the "Wehrmacht" armed forces from 1936, it was strongly affected by strategic bombing during World War II.