Following World War I and the Treaty of Versailles, the German protectorate of New Guinea was divided up amongst the victorious Entente powers. The southern part of the protectorate, comprising territory on New Guinea and islands south of the equator, came under the Australian Mandate. New Zealand was awarded German Samoa (Western Samoa). The Empire of Japan, which had allied with the Entente powers in 1914 and secured the sea lanes in the West Pacific and Indian Oceans against the German Navy, was mandated the northern part of the protectorate, comprising the Micronesian islands north of the equator. Under the terms of the South Pacific Mandate, it was required that the islands were demilitarized and that Japanese influence should not be further expanded in the Pacific. Despite the terms of the mandate, which supported trade and migration, the Japanese governed their islands as colonial possessions, ensuring that they were off-limits to foreigners. The Mandate, while not economically productive for Japan, was highly strategic, since the islands dominated the Pacific sea-lanes. From the 1930s onward the Japanese Navy began to construct airfields, fortifications and ports on the islands, which became important military staging posts during World War II.
— OR —