As the Finnish Civil War begun, Rahja was wounded in the Battle of Kämärä and stayed in the hospital until the end of war in May 1918. Following the defeat, Rahja fled to the Russian SFSR and became a founding member of the Finnish Communist Party (SKP). He was a delegate for the SKP and the First and Second congresses of the Third International.
As the Finnish Civil War commenced, Rahja was injured in the Battle of Kämärä and remained in the hospital until the conclusion of the war in May 1918. Following the defeat, Rahja escaped to the Russian SFSR and became a founding member of the Finnish Communist Party (SKP). He served as a delegate for the SKP and attended the First and Second congresses of the Third International.