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.