After returning to Finland, Wallenius took part in the Finnish Civil War of 1918 on the side of the anti-Communist Whites. He commanded a platoon in Tervola and Tornio. In Lapland Wallenius met reindeer herder Aleksi Hihnavaara, with whom he travelled around Lapland, and the two became good friends. Wallenius was later appointed commander of the troops around Kuolajärvi and Kuusamo. Here he was given the command of the northern group in the failed March 1918 Viena expedition, which tried to annex parts of East Karelia to Finland. He was also the leader of the second Petsamo Expedition in May 1920.