Formally separated from German East Africa in 1890, it had become fully independent in 1963, with responsibility for its own defense and foreign affairs, as a result of Britain giving up its protectorate over it.

In a series of parliamentary elections preceding this change, the Arab minority succeeded in retaining the hold on power it had inherited from Zanzibar's former existence as an overseas territory of Oman.

Frustrated by lack of parliamentary representation compared to its share of votes in the July 1963 election, the African Afro-Shirazi Party (ASP), led by John Okello, mobilised around 600–800 men on the main island of Unguja on the morning of 12 January 1964.

The Eastern Bloc powers of East Germany and the Soviet Union, along with the anti-Soviet People's Republic of China, immediately recognised the new government and sent advisors.

[9] Zanzibar had a population of around 230,000 Africans—some of whom claimed Persian ancestry and were known locally as Shirazis[10]—and also contained significant minorities of 50,000 Arabs and 20,000 South Asians, who had long been prominent in business and trade.

[9] To maintain control, the coalition government banned the more radical opposition parties, filled the civil service with its own appointees, and politicised the police.

[14] Due to the layout of the constituencies, which were gerrymandered by the ZNP, the ASP, led by Abeid Amani Karume, won 54 percent of the popular vote but only 13 seats.

[11] The Umma Party, formed that year by disaffected radical Arab socialist supporters of the ZNP,[16] was banned, and all policemen of African mainland origin were dismissed.

[15][17] This removed a large portion of the only security force on the island, and created an angry group of paramilitary-trained men with knowledge of police buildings, equipment and procedures.

[18] The new Arab-dominated government made it clear that in foreign policy, the Sultanate of Zanzibar would be seeking close links with the Arab world, especially Egypt, and had no interest in forging relationships with the nations on the African mainland, as the black majority wished.

[29] Within six hours of the outbreak of hostilities, the town's telegraph office and main government buildings were under revolutionary control, and the island's only airstrip was captured at 2:18 pm.

As claimed by Sullivan, he marched his entire force (not one policeman had been killed or wounded) down to the Stone Town wharf to board some boats that took them out to a ship, the Salama, to take them away from Zanzibar.

[3] Sixty-one American citizens, including 16 men staffing a NASA satellite tracking station, sought sanctuary in the English Club in Zanzibar Town, and four US journalists were detained by the island's new government.

[33] Those travelling in the car convoy to the English Club were shocked to see the battered bodies of Arab men lying out on the streets of Stone Town with their severed penises and testicles shoved into their mouths.

[34] As part of Okello's carefully laid out plans, all over the island, gangs of Africans armed with knives, spears and pangas (machetes) went about systematically killing all the Arabs and South Asians they could find.

[37] Okello was not widely known in Zanzibar, and the government was more concerned with monitoring the ASP and Umma rather than a little-known and barely literate house painter and minor union official.

[3] Seeking to distance himself from the volatile Okello, Karume quietly sidelined him from the political scene, although allowing him to retain his self-bestowed title of field marshal.

[1][32] Okello claimed in radio speeches to have killed or imprisoned tens of thousands of his "enemies and stooges",[1] but estimates of the number of deaths vary greatly, from "hundreds" to 20,000.

Okello formed the Freedom Military Force (FMF), a paramilitary unit made up of his supporters, which patrolled the streets and looted Arab property.

[48][49] The behaviour of Okello's supporters, his violent rhetoric, Ugandan accent, and Christian beliefs were alienating many in the largely moderate Zanzibari and Muslim ASP.

[52] The merger was seen by contemporary media as a means of preventing communist subversion of Zanzibar; at least one historian states that it may have been an attempt by Karume, a moderate socialist, to limit the influence of the radically left-wing Umma Party.

[45] British military forces in Kenya were made aware of the revolution at 4:45 am on 12 January, and following a request from the Sultan were put on 15 minutes' standby to conduct an assault on Zanzibar's airfield.

[56] The Manley docked at Zanzibar Town harbour, but the US had not sought the Revolutionary Council's permission for the evacuation, and the ship was met by a group of armed men.

[1] Dissatisfied with their low pay rates and with the slow progress of the replacement of their British officers with Africans,[63] the soldiers' mutiny sparked similar uprisings in both Uganda and Kenya.

[57] A partial evacuation of British citizens was completed by 17 January,[68] when the army riots in Southeast Africa prompted Rhyl's diversion to Tanganyika so that the troops she was carrying could assist in quelling the mutiny.

[70] Instead Operation Finery was drawn up, which would involve a helicopter assault by Royal Marines from HMS Bulwark, a commando carrier then stationed in the Middle East.

[53] With the merger of Tanganyika and Zanzibar on 23 April, there were concerns that the Umma Party would stage a coup; Operation Shed was designed to provide for intervention should this happen.

[71] Concern over a possible coup remained though, and around 23 September Shed was replaced with Plan Giralda, involving the use of British troops from Aden and the Far East, to be enacted if the Umma Party attempted to overthrow President Julius Nyerere of Tanzania.

[72] An infantry battalion, tactical headquarters unit and elements of the Royal Marines would have been shipped to Zanzibar to launch an amphibious assault, supported by follow-on troops from British bases in Kenya or Aden to maintain law and order.

[75] The revolutionary government also instituted social reforms such as free healthcare and opening up the education system to African students (who had occupied only 12 per cent of secondary school places before the revolution).