At the beginning of the First World War, Armin and the IV Corps were a part of the 1st Army on the Western Front, where they fought in the trench warfare that defined the first years of the conflict. For his handling of combat operations on the Western Front, particularly at Arras and on the Somme, he was awarded the Pour le Mérite in 1916. The following year he was appointed commander of the 4th Army and also served as commander-in-chief in the Flanders region. During his time as commander, the 4th Army withstood several attacks from British and French armies, notably the Third Battle of Ypres. For his performance as commander, Armin was awarded the Order of the Black Eagle, as well as the oak leaf cluster to the Pour le Mérite. Armin was still in command of the 4th Army during the Spring Offensive of 1918. On 25 April, his troops captured the Kemmelberg, although they were later forced to retreat to the Antwerp-Maas defensive line. With the signing of the Armistice on 11 November, Armin took command of Army Group A and returned with it to Germany, where, following the demobilisation of his troops, he resigned.