Nauru was occupied by Australian troops in 1914 (at the outbreak of WWI) and formally became a League of Nations Class C Mandate in 1920, with Australia as the administering power on behalf of itself, the UK and New Zealand (the atlas's existing 'german_new_guinea' record already covers Nauru's earlier 1906-1919 German annexation and Australian wartime occupation, so this record begins at the 1920 formal mandate to avoid overlap). Phosphate mining, begun under German rule, continued and intensified as the territory's economic mainstay. Occupied by Japan 25 August 1942-13 September 1945 (WWII), after which Australian administration resumed as a UN Trust Territory from 1947 (Australia, UK and NZ as joint trustees).
Achieved self-governance in January 1966; the Nauru Independence Act 1967 (Australian Parliament) took effect 31 January 1968, making Nauru fully independent -- the polity already in the atlas as the modern 'nauru' record (Wikidata Q697), which the atlas dates from 1968.
Open Library-verified (ISBN 9780195532890); a legal/historical study specifically of Nauru's mandate and trusteeship era and the phosphate mining that defined it -- directly on-topic for this exact record, by a scholar (later ICJ judge) who represented Nauru in its case against Australia over rehabilitation of mined lands.