During World War II, and after the fall of Greece to the Nazis, he was active in the Greek Resistance, joining the left-wing National Liberation Front (EAM) and serving as an officer in the Greek People's Liberation Navy (ELAN), the naval branch of the more famous Greek People's Liberation Army (ELAS). He was also involved in politics, being elected in the "National Council" that EAM had set up, representing Edessa. His father Neokosmos Grigoriadis was Chairman of the Council.
