Long before European colonialism, Arab and Persian traders operated extensive slave routes from East Africa across the Indian Ocean — a trade that European powers later expanded dramatically. From approximately 650 CE onward, enslaved Africans were transported from the Swahili Coast (present-day Tanzania, Kenya, Mozambique) through Zanzibar, and onward to the Arabian Peninsula, Persia, India, and as far as Southeast Asia and China. The island of Zanzibar became the world's largest slave market by the 19th century, with an estimated 50,000 enslaved people passing through annually. The Omani Sultanate, which controlled Zanzibar from 1698, made the slave trade the economic backbone of its empire. When Britain finally signed an anti-slave-trade treaty with Zanzibar's Sultan Barghash in 1873 under diplomatic pressure, the slave market was formally closed — but illegal trade continued for decades. Historians estimate between 2 million and 17 million enslaved Africans were sold through Indian Ocean routes over 1,200 years. The Portuguese, who colonized the East African coast from the 15th century, both participated in and disrupted existing Arab slave routes, eventually redirecting enslaved Africans toward Brazil. Unlike the transatlantic trade, the Indian Ocean slave trade has received comparatively little scholarly or public attention.