As the Civil War broke out in January 1918, Lassy offered his services for the Peoples Delegation and was hired as a translator by Yrjö Sirola. According to Lassy, he joined the Reds because he saw the worker's revolution as a subject on anthropological research, but was soon sucked into the aspect as he learned to understand their demands. After the Battle of Helsinki, Lassy was captured by the Whites and given a 9-year-sentence for treason but he was pardoned in November 1918. At the Suomenlinna prison camp, Lassy was introduced to socialism by his cellmates Sulo Wuolijoki and Taavi Tainio.