Aarnio was a volunteer with the White Guards at the Savonia Front during the Finnish Civil War. Later he participated in the Estonian War of Independence, the Latvian War of Independence and the Aunus expedition in East Karelia. Aarnio transferred from active military service to the reserve officer corps in 1920. He graduated from the Kadettikoulu in 1926 and the Sotakorkeakoulu in 1933. Aarnio was promoted as captain in 1929 and served in the Foreign relations department of the General Staff.