Following the First World War, the Union of South Africa was granted the administration of South West Africa (formerly a German colony and today the sovereign state of Namibia) as a League of Nations mandate, which became treated in most respects as if it were another province of the Union, but never was formally annexed.
The Union of South Africa was a self-governing autonomous dominion of the British Empire. Its independence from the UK was confirmed in the Balfour Declaration 1926 and the Statute of Westminster 1931. It was governed under a form of constitutional monarchy, with the Crown represented by a governor-general. The Union came to an end with the enactment of a new constitution on 31 May 1961, by which it became a republic and temporarily left the Commonwealth, under the new name Republic of South Africa.
Unlike Canada and Australia, the Union of South Africa was a unitary state, rather than a federation, with each colony's parliaments being abolished and replaced with provincial councils. A bicameral parliament was created, consisting of a House of Assembly and Senate, and its members were elected mostly by the country's white minority. During the course of the Union the franchise changed on several occasions always to suit the needs of the government of the day. Parliamentary supremacy was a convention of the constitution, inherited from the United Kingdom; save for procedural safeguards in respect of the entrenched sections of franchise and language, the courts were unable to intervene in Parliament's decisions.
Owing to disagreements over where the Union's capital should be, a compromise was reached in which every province would be dealt a share of the benefits of the capital: the administration would be seated in Pretoria (Transvaal), Parliament would be in Cape Town (Cape Province), the Appellate Division would be in Bloemfontein (Orange Free State). Bloemfontein and Pietermaritzburg (Natal) were given financial compensation.
The Union initially remained under the British Crown as a self-governing dominion of the British Empire. With the passage of the Statute of Westminster in 1931, the Union and other dominions became equal in status to the United Kingdom and it could no longer legislate on behalf of them. The Monarch was represented in South Africa by a Governor-General, while effective power was exercised by the Executive Council, headed by the Prime Minister. Louis Botha, formerly a Boer general, was appointed first Prime Minister of the Union, heading a coalition representing the white Afrikaner and English-speaking British diaspora communities. Prosecutions before courts were instituted in the name of the Crown (cited in the format Rex v Accused) and government officials served in the name of the Crown.