This article is from the South Africa FAQ, by Scott Hazelhurst scott@cs.wits.ac.za with numerous contributions by others.
When did South Africa gain independence?
Summary: The Union of South Africa formed in 1910, and although
formally the British government and parliament had reserve powers, de
facto independence dates from 31 May 1910. Formally, South Africa's
full sovereignty came after the passage of the Statute of Westminster
in 1931. (South Africa became a republic in 1961, but this only had
the effect of changing the head of state and did not affect the
independence/sovereignty issue.)
At the conclusion of the Anglo-Boer war in 1902, the British had
control over the entire area of South Africa:
* the Cape and Natal which were self-governing colonies
* the Transvaal and Orange Free State which were run directly by
Britain
* the "High Commission Territories" or "protectorates" of
Bechuanaland (now Botswana), Basutholand (now Lesotho), and
Swaziland.
In 1906 and 1907 the Transvaal and the Free State were given
responsible government.
After constitutional negotiations culminating in a National Convention
(excluding blacks), the Union of South Africa was formed in 1910 (the
Union excluded the High Commission Terrritories, although
constitutional provision was made for their entry).
Union of South Africa
At Union, South Africa (like Australia, Canada and New Zealand at that
stage) was not strictly an independent country. Legislative powers
were vested in the South African Parliament (a House of Assembly and
Senate) and effective executive control through South African
ministers (nominally vested in the King through the Governor-General).
However, the Governor-General had the power to reserve signing bills,
and all South African laws were subject to the British Colonial Laws
Validation Act. (I believe these powers were never used and in
practice, independence dates from 1910).
In the Transvaal, Free State and Natal only whites could vote, while
in the Cape there was a formally non-racial franchise (a right later
weakened, and then finally removed in the 1960s).
After the First World War, the status of the dominions like South
Africa changed and a report of the Imperial Conference declared that
the dominions were equal in status and in now way subordinate to
Britain. The Statute of Westminster 1931 (can be found as:
gopher://freenet.victoria.bc.ca/0/archives/history/etext/statute.westm
inster.1931) put legal effect to this when the British Parliament gave
up its right to legislate for the dominions. This was recognised in
South African legislation by the Status of the Union Act 1934 (can be
found as:
ftp://ftp.cs.ubc.ca/pub/local/FAQ/african/sa/statusunionact.txt) which
declared the Union a "sovereign independent state". (So, although
there were strong political links with Britain and although South
Africa and Britain shared a common king, South Africa was an
independent country by the start of the Second World War).
Republic of South Africa
After a 1960 referendum among white voters, South Africa became a
republic on 31 May 1961, and allowed its membership of the
Commonwealth to cease after protests from other Commonwealth members.
The structure of the constitution remained the same, with an
indirectly-elected State President peforming the role of head of state
(in place of the King/Governor-General), and the prime minister
remaining head of government.
In 1983 a new constitution was adopted and inaugurated in 1984. The
offices of prime minister and state president were combined into the
State President's office. Parliament was restructured to comprise a
House of Assembly (whites), a House of Representatives (coloureds),
and House of Delegates (Indians). A Presidents' Council (with
effective white control) was set up as a deadlock breaking mechanism.
 
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