On 26th April, 1964, the United Republic of Tanganyika and Zanzibar was born. In October of that year the country was renamed the United Republic of Tanzania (a portmanteau of Tanganyika and Zanzibar).
In December 1961, the East African state of Tanganyika achieved independence from the British, as a constitutional monarchy under a sultan, who was removed from power in June of the next year when the country became a republic. The British military had administered Tanganyika since the end of the First World War, before that it had been under the colonial control of the German East Africa Company. Prior to that, the Sultan of Oman ruled the area after he helped the indigenous population drive Portuguese colonists from the coast.
The Sultan of Oman also ruled the archipelago of Zanzibar from where he also forced the withdrawal of the Portuguese. In the nineteenth-century the islands became a British protectorate under the Sultan of Zanzibar. The British protection was part of their campaign against the slave trade, the islands’ ports being an important to the trade. In December 1963, Britain granted Zanzibar full independence as a constitutional monarchy. Like Tanganyika, it became a republic, but only after a bloody revolution that cost thousands of lives.
See the U.S. Department of State’s website for information about Tanzania, including the history of its constituent parts.