On linguistic and archaeological grounds, it is likely that voyagers from southern Melanesia arrived in the Gilberts long before A . D . 600, the earliest radiocarbon date obtained up to now. Kiribati language and culture show signs of borrowing from western Polynesia at some time after the islands were settled. The political and social structure of all the islands except for Butaritari-Makin and Banaba was forcibly unified, possibly in the seventeenth century, when armies led by Kaitu, Beru, and Uakeia of Nikunau introduced the meetinghouse organization. Regular contacts with Europeans and Americans began when merchant ships sailing new routes across the Pacific, New England whalers, and exploring expeditions discovered or rediscovered all the islands between 1765 and 1826. Resident traders bought coconut oil from 1846 to the 1870s and then switched to copra, which remains Kiribati's sole agricultural export. A British protectorate was proclaimed over the Gilberts and their Polynesian neighbors, the Ellice Islands, in 1892. The Japanese occupation of the Gilberts early in World War II ended with an American victory in the "particularly bloody battle at Tarawa" (as Richard Overy has aptly termed it) fought in November 1943. The phosphate mine on Banaba provided most of the colony's revenue and employment for its people from 1900 until the deposits were exhausted in the year of independence; I-Kiribati still mine phosphate on the neighboring independent island of Nauru. Since 1967, the Marine Training School has made it possible for many young men to get jobs as seamen on West German ships and to add greatly to their families' incomes through remittances. Four years after the Ellice Islands had separated from the colony to become the state of Tuvalu, the Gilberts also became independent as the Republic of Kiribati on 12 July 1979.