As a young man he became a supporter of Finnish independence. His father changed the original, Swedish, family name Johansson to Airo (lit. "oar"). During the Civil War in Finland (1918), Airo served in the artillery on the White side, taking part in battles near Viipuri. At the end of the war he was a lieutenant. Afterwards he was trained as an officer in Lappeenranta artillery school and was sponsored to the French military academy, École militaire in St.-Cyr in 1920. In 1921 he was accepted into École Supérieure de Guerre, the French officer training academy, from which he graduated as a captain in 1923, at the age of 27. Mannerheim invited him to join Finland's Defense Council as a secretary.