The politicking behind the scenes for the formation of the Union of South Africa allowed the foundations of Apartheid to be laid.
On 31 May 1910 the Union of South Africa was formed under British dominion. It was exactly eight years after the signing of the Treaty of Vereeniging, which had brought the Second Anglo-Boer War to an end. Each of the four unified states was allowed to keep its existing franchise qualifications and Cape Colony was the only one which permitted voting by (property owning) non-whites.
Whilst is it argued that Britain hoped that the 'non-racial' franchise contained in the constitution courtesy of the Cape would eventually be extended to the whole of the Union, it is hardly likely that this was truly believed possible. (A delegation of white and black liberals traveled to London, under the leadership of the former Cape prime minister William Schreiner, to protest against the colour bar enshrined in the new constitution.) The British government was far more interested in creating a unified country within its Empire; one which could support and defend itself. A union, rather than a federalized country, was more agreeable to the Afrikaner electorate since it would give the country a greater freedom from Britain. (Louis Botha and Jan Christiaan Smuts, both highly influential within the Afrikaner community, were closely involved in the development of the new constitution.) It was necessary to have Afrikaner and English working together, especially following the slightly acrimonious end to the war, and the satisfactory compromise had taken the last eight years to reach. Written into the new constitution, however, was a requirement that a two-thirds majority of parliament would be necessary to make any changes.
The British High Commission Territories of Basutoland (now Lesotho), Bechuanaland (now Botswana), and Swaziland were excluded from the Union precisely because the British government was worried about the status of the indigenous populations under the new constitution. It was hoped that, at some time in the (near) future, the political situation would be right for their incorporation. (In fact, the only country which may have been considered for inclusion was Southern Rhodesia, but the Union had become so strong that white Rhodesians quickly rejected the concept.)
Although not truly independent, most historians1, especially those in South Africa, consider 31 May 1910 to be the most appropriate date to be commemorated. South Africa's independence within the Commonwealth of Nations was not officially recognized by Britain until the Statute of Westminster in 1931, and it was not until 1961 that South Africa became a truly independent republic.
1 Africa since 1935, Vol VIII of the UNESCO General History of Africa, published by James Currey, 1999, editor Ali Mazrui, p108.