Upon the annexation of Hanover by Prussia in 1866 following the Austro-Prussian War Siemens became a Prussian citizen and liable to conscription. He was conscripted in 1870 as a private to fight in the Franco-Prussian War where he was wounded at the Battle of Beaune-la-Rolande. It was for his actions in this battle where Prussian forces won a decisive victory over the numerically superior French army that he was awarded the Iron Cross. After demobilization in 1871 he returned to the family business in Woolwich and assisted with the building of furnaces for use in steel foundries and crematoria.