In August 1936, he was arrested by the State Security of the Ioannis Metaxas regime and imprisoned. From prison, he issued a letter urging all Greeks to resist the Italian invasion of October 1940 and transform the war into an anti-fascist war. Some KKE cadre, who did not believe that the ongoing war between the big imperialist powers differed from the First World War due to the existence of the USSR on the world scene, considered that this letter was fabricated by the Metaxas regime. Zachariadis was even accused of releasing it to win the favor of Konstantinos Maniadakis and be released from prison. Zachariadis's letter remains a cornerstone of KKE's vital contribution to the National Resistance movement against the Fascist occupiers (1941-1944).