Rubuta ci gaba ga wannan sakin layi - Aarnio ya kasance mai ba da agaji tare da White Guards a gaban Savonia a lokacin Yakin basasa na Finnish. Daga baya ya shiga cikin Yakin 'Yancin Estonia, Yakin' Yancin Latvia da kuma tafiyar Aunus
a Gabashin Karelia. Aarnio ya canja daga aikin soja na aiki zuwa jami'in jami'in ajiya a shekarar 1920. Ya kammala karatunsa daga Kadettikoulu a 1926 da Sotakorkeakoulu a 1933. Aarnio ya zama kyaftin a shekarar 1929 kuma ya yi aiki a sashen hulda da kasashen waje na Janar Janar.