Between 1962 and 1965 Britain engaged in covert operations in support of Royalist forces fighting the Egyptian backed Republican regime that had seized power in the Yemeni capital Sana'a in September 1962. Covert action was regarded as a legitimate tool of foreign policy as Britain attempted to secure the future of the newly formed South Arabian Federation against the animus of Nasser. The use of covert action, as well as the quasi approval given to the use of mercenaries to support the Royalist cause, was the inevitable result of policy differences within Whitehall (most notably between the mandarins' of the Colonial Office and the Foreign Office) as well as international constraints imposed upon the UK in the aftermath of the Suez crisis. The book examines the extent to which British policy, while successful in imposing a war of attrition upon Nasser in the Yemen, contributed to the political demise of the very objective covert action was designed to secure: the future stability of the Federation of South Arabia. The study makes extensive use of primary sources in producing the first detailed account of British involvement in the Yemen Civil War, and how the experience shaped British foreign policy. It breaks new ground by analyzing the extent to which Britain came to support the Royalist cause despite public declarations of non-involvement in the Yemen conflict, and details for the first time how London's tacit support for mercenary operations' in the Yemen came to enlist the help of Saudi Arabia and Israel.