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 started, Rahja was injured in the Battle of Kämärä and stayed in the hospital until the end 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), playing an important role in the organization's activities.