Complex sentence: The founder and leader of the organization was Georgios Grivas, a lieutenant colonel of the Hellenic Army at the time and leiter, during' 50s, the military leader of EOKA, during the guerrilla campaign of Greek Cypriots against British colonial rule.
Split into simpler sentences: The founder and leader of the organization was Georgios Grivas, a lieutenant colonel of the Hellenic Army at the time. Leiter, during' 50s, he was the military leader of EOKA, during the guerrilla campaign of Greek Cypriots against British colonial rule.