31 May 1910
Following delegates of the Boer Republics' agreement by 54 to 6, to accept the British proposals for peace on 31 May 1902, the Treaty of Vereeniging was finally signed by representatives of both sides at Melrose House, Pretoria. The treaty paved the way for the establishment of the Union of South Africa, which was inaugurated eight years later. The Union was made from four colonies: Cape Colony , Natal, Orange River Colony and the Transvaal.
References

Evans, A.B., Union of South Africa, from African History, [online], Available at www.africanhistory.about.com [Accessed 31 May 2013]