The inevitability of UDI (click HERE for a copy) seems to have been accepted, the major British worry apparently being the economic impact on Britain, and stability of neighbouring African countries; the awareness of the economic power wielded through Portuguese control of Mozambican copper exports on the economies of both Britain and Zambia. The fear that British troops may have been sent in to restore order was quickly ruled out, one civil servant saying "we don't do that sort of thing against white ex-colonials". The British government apparently decided against the use of force even before UDI, partly due to the number of Rhodesian-born officers in the RAF in the mid-1960s (even if it were assumed that little, or no resistance would have been offered,  

despite the perception that Rhodesia had "the strongest air force in Africa", with the possession of airbases which were seen as an indispensable part of international security). It was also felt that the possibility of Rhodesian action against Zambia before any international forces were able to maintain order further mitigated the chance of a successful operation. 

How far Ian Smith was gambling that the initially lack-lustre approach by the British government would eventually move from a tacit acceptance that the change was inevitable, and then to support, is uncertain. However, the South Africans - even though there remained a degree of resentment with Rhodesia, aided by the (English, not Afrikaans) language differences and Rhodesian enmity during the Boer War, were to prove a more reliable ally. From the very start in the early 1960s, when ANC cadres were detected trying to cross the border between Northern and Southern Rhodesia, before heading towards South Africa, Rhodesia was seen very much as the first line of defence.