In the Austro-Prussian War Hassel took part, as a member of the in the Battles of Hühnerwasser and the Münchengrätz. On 28 July 1866 he became a Captain and was decorated with the Order of the Crown. In March 1870, now a Company commander, Hasel was transferred again, this time to the 35th Brandenburg Fusilier regiment. With the mobilisation that year against France, he joined the General Staff of the 16th Division. In this position he participated in the Battles of Spicheren, Mars-la-Tour, Gravelotte, Amiens, Hallue and St. Quentin. He also took part in the successful Siege of Metz. Hassel was promoted again on 22 December 1870, now becoming a Major, and awarded the Iron Cross. After the war was formally concluded by the Treaty of Frankfurt (1871), he was transferred to the General Staff of the 8th Army Corps.