Otto enlisted in January 1869 in a Regiment Prussian Grenadiers at Altona. The following year he participated in the Franco-Prussian War, integrating the Prussian forces that carried out the campaigns in France that year. On 16 August he was present with his regiment at the Battle of Mars-la-Tour, a violent engagement in which a part of the Prussian Army successfully intercepted the French Army of the Rhine that was trying to reach Verdun, but at the cost of many casualties. In this bloody battle, Otto was wounded, while his chief, a colonel surnamed von Schöning, was killed. Due to his brave conduct in battle, Otto was promoted to standard-bearer and then, in September, to second lieutenant, also receiving the Iron Cross Second Class. Later he also received the &lt;a href="War%20Commemorative%20Medal%20of%201870/71"&gt;War Commemorative Medal of 1870/71&lt;/a&gt;.