When the Austro-Prussian War started John remained chief of staff of the South Army in Italy under the command of Archduke Albrecht. During the campaign he distinguished himself at the Battle of Custozza (24 June 1866) and was promoted "Feldmarshall-Leutnant" on the battlefield. After the defeat of Benedek's North Armt at Königgrätz, he accompanied the Archduke to the northern theater of war.