The Zanzibar Revolution occurred on 12 January 1964, when 600–800 mainly African men, led by John Okello and supported by the Afro-Shirazi and Umma Parties, overthrew Sultan Jamshid bin Abdullah and his largely Arab government.

[1][3] The ASP, led by Abeid Karume, and the Umma Party, under Abdulrahman Mohamed Babu, formed the Revolutionary Council to govern the country.

Following these events, several western governments, who suspected the revolutionaries had the backing of Cuba, the People's Republic of China and other communist countries, made plans for the evacuation of their citizens.

The arrival of HMS Rhyl caused some concern to the revolutionary government, as she carried a company of troops from the first battalion of the Staffordshire Regiment, who had been sent to Zanzibar from Kenya due to inaccurate reports that security there was deteriorating rapidly.

The Hebe exacerbated the situation, as she had just finished removing stores from the naval depot at Mombassa and was loaded with weapons and explosives; as a result, the Royal Navy refused to allow representatives from the Zanzibari government on board to search the ship, prompting rumours that she was an amphibious assault vessel.

[10] On 30 January, the British Commanders Committee of East Africa authorised Operation Parthenon, whose objective was to restore law and order in Zanzibar should the Revolutionary Council fail to do so.

[11] Specifically, there was a concern that the radical left-wing Umma Party, supported by Okello's armed militia, would oust the more moderate members of the ASP from government and seize control.

[17] Finery would have involved a helicopter assault on Unguja, the main base of revolutionary power, by Royal Marines from the commando carrier HMS Bulwark.

To provide a more immediate response, plans were put in place for a smaller scale operation, which could be launched within 24 hours, should evacuation of remaining British citizens in Zanzibar be required.

[17] The merger of Zanzibar with Tanganyika to form Tanzania on 23 April may have provided the catalyst for the Umma Party to attempt a coup, and so from around this date, Finery was supported by Operation Shed: an airlift of a battalion of troops, accompanied by scout cars, to seize Unguja's airfield and protect Abeid Karume's government.

Shed would have entailed the airlift of a battalion of British troops to Unguja, the main base of revolutionary power, to provide support to Karume and any forces loyal to him.

[20] In light of this information, Shed was modified on 9 June to an airborne assault by Royal Marine commandos from HMS Centaur, which would then be supported by the landing of the battalion and armoured cars from Kenya.

[22] In January, prior to the merger, Nyerere had requested British military assistance to quell a mutiny in Tanganyika,[8] as he had spent two days hiding in a Catholic mission after the mutineers raided the State House.

[26] By the autumn of 1964, Western interests in Zanzibar were practically non-existent, and in October, the British Chiefs of Staff were informed that Nyerere was very unlikely to request intervention, and as a result, the plan was suspended.