Over the centuries, the islands were invaded by a succession of diverse groups from the coast of Africa, the Persian Gulf, Indonesia, and Madagascar. Portuguese explorers visited the archipelago in 1505. "Shirazi" Arab migrants introduced Islam at about the same time. Between 1841 and 1912, France established colonial rule over Grande Comore, Anjouan, Mayotte, and Moheli and placed the islands under the administration of the governor general of Madagascar. Later, French settlers, French-owned companies, and wealthy Arab merchants established a plantation-based economy that now uses about one-third of the land for export crops. After World War II, the islands became a French overseas territory and were represented in France's National Assembly. Internal political autonomy was granted in 1961. Agreement was reached with France in 1973 for Comoros to become independent in 1978. On July 6, 1975, however, the Comorian Parliament passed a resolution declaring unilateral independence. The deputies of Mayotte abstained. As a result, the Comorian Government has effective control over only Grande Comore, Anjouan, and Moheli. Mayotte remains under French administration.