Kubiliūnas ya kammala karatunsa daga Realgymnasium a Riga kuma ya ci gaba da karatunsa a Makarantar Soja ta Vilnius, inda ya kammala karatunsa a watan Disamba na 1914. A lokacin Yaƙin Duniya na I, ya yi aiki a rundunar sojojin Rasha a Gabashin Gabas. An sanya shi zuwa 2nd Riga Battalion na Latvian Riflemen. An ba shi lambobin Rasha da yawa, gami da Gicciye na St. George da haɓaka zuwa kyaftin don ayyukansa a lokacin Yaƙin Kirsimeti
a shekara ta 1916.