The Union of South Africa Act had the effect of uniting South Africa but of dividing southern Africa. The partitioning of the whole area, with imperial responsibility retained in the Rhodesias and Nyasaland, and in the High Commission Territories of Basutoland, Bechuanaland, and Swaziland, was not intended to be a permanent arrangement. The Union as formed was expressly regarded both in London and Pretoria as a provisional union. The Act of 1909 laid down a procedure for the incorporation of Rhodesia, and the Schedule prescribed the terms for a possible future transfer of administration in the High Commission Territories. However, the expectations of 1908–10 were not realised, and the formal political expansion of the Union in southern Africa did not materialise.

Perhaps the fundamental reason for this ‘unconsummated Union’ was determined in 1908. The British government's decision to retain control of the High Commission Territories after Union meant that the first setback to dreams of a Greater South Africa were registered even before the Union came into being. This important decision conflicted with the known wishes of the white South African leaders. The general strategy of the British government in 1908–9 was well expressed by Winston Churchill, who had just moved from the Colonial Office to the Board of Trade. He wrote to the colonial secretary, Lord Crewe: