Zanzibar is 95% Islamic in religion, and has been so for hundreds of years. It was once part of the Persian Empire, then under the Caliphate of Oman. It was ruled by a Sultan. Zanzibar was conquered by the British in the late 19th century. The islands were involved in the Arab slave trade, and the British conquest was an attempt to stop this. They appointed puppet rulers, and Zanzibar became a 'protectorate', not a colony.