In 1868 he became inspector of the Mainz garrison and lieutenant general. After the beginning of the Franco-Prussian War, Kummer was given command of the 3rd Reserve Division on August 10, 1870, with which he participated in the Siege of Metz until its surrender. His division fought in the Battle of Noisseville and carried the brunt of the Battle of Bellevue. After the fall of Metz on October 27, 1870, he was in command of Metz until November 6. His Landsturm Division was tasked with transporting the prisoners away and then stayed at home. Instead, Kummer took over the 15th division on October 27, 1870. This was part of the VIII Army Corps (First Army) in northern France. He fought there successfully in the Battle of Amiens and the Battle of the Hallue and led the advance on Bapaume, which led to the Battle of Bapaume. After the victory at Saint-Quentin on January 19, 1871, the fighting was over for him. He had already received the oak leaves for the Pour le Mérite on January 12, 1871.