In 1918, Finland declared its independence, and the Finnish Civil War erupted. Hallamaa joined the "whites" in Seinäjoki. Here he was assigned to the "Uudenmaan rakunapataljoona" unit where he fought in the battles at Väärinmaja, Kuhmoinen, Tampere and Lempäälä. Hallamaa, now a lance corporal ("korpraali"), was transferred to the navy after the war, where he worked as a signaller. Later that year he was promoted to corporal ("alikersantti") and assigned to Gogland where he was tasked with gathering information on naval traffic in the Gulf of Finland and Red Navy movements in particular. He was listening to Soviet radio traffic along with Ragnvald Lind (later Ragnvalt Lautakari) who would continue to work in the radio intelligence under Hallamaa's lead. Hallamaa even managed to decipher some of the codes sent out by the Soviets. This awoke the interest in the higher military circles and Hallamaa was invited to come and hold a presentation on the work done at Gogland. Soon after, Hallamaa was promoted to the real of sergeant. He was now tasked with training radio operators and sent on trips to Kotka and Turku to try to repair radio stations that had been left there by the Russians.