Japan's League of Nations Class C mandate over the former German-held islands of Micronesia north of the equator -- the Caroline, Marshall, and Mariana Islands (excluding US-held Guam). Awarded to Japan on 28 June 1919 after its WWI occupation of German New Guinea's Pacific holdings (seized by the Imperial Japanese Navy by October 1914), administered from Koror (Palau) by the Nan'yo-cho (South Seas Bureau) with heavy Japanese civilian settlement -- by 1939 the roughly 77,000 Japanese settlers (of a total 129,104) outnumbered the c. 51,700 indigenous Micronesians, and Saipan was already about 90% Japanese by 1937.
Conquered island by island by US forces in 1943-1945 (Tarawa, Kwajalein, Saipan, Peleliu among the campaigns); Japanese administration collapsed with Japan's surrender in 1945, and US military government took over the islands immediately, though the League of Nations mandate was not formally revoked by the United Nations until 18 July 1947 (UN Security Council Resolution 21), when the territory was reconstituted as the US-administered Trust Territory of the Pacific Islands. End year here is set to 1945 (de facto collapse of Japanese control) rather than the 1947 de jure UN revocation date -- flagged for human review, see flags_for_human.
The standard English-language monograph on Japanese colonial rule in Micronesia; covers administration, settlement, economy, and the wartime endgame in depth. Confirmed on Open Library (also listed in the Wikipedia article's own bibliography).