When World War II reached Norway with the Nazi German invasion, Siem fought for his country in the Battle of Hegra Fortress. After the conventional Norwegian armed forces capitulated, Siem returned to his job at Akers Mekaniske Verksted. While working there he became an informant for the secret intelligence organization RMO, which was established in 1942. His information on the Port of Oslo became especially important for Norwegian ship saboteurs, such as Max Manus, Gregers Gram and Roy Nielsen. When the leader of RMO, A.K. Rygg, had to flee the country in the autumn 1943, Siem took over the leadership together with Sigurd Sverdrup. Among the results of Siem's information was the sabotage towards SS "Donau".