17 December 1920
The League of Nations, the world body formed after World War 1 in 1919, afforded South Africa a mandate to govern South-West Africa (now Namibia). The mandate gave South Africa full power of administration and legislation over the territory as an integral part of the Union of South Africa, with the aim of promoting the well-being and development of the inhabitants. In 1946, the Legislative Assembly of South-West Africa in Windhoek requested unanimously to be incorporated into the Union, a decision fully endorsed by South Africa. This request was rejected by the United Nations (UN), the new international body which was formed after World War 2. The UN advised that the territory should be put under UN trusteeship, according to the provisions of Chapter XII of the Charter. The Union of South Africa refused to comply and thus a checkmate situation was reached that was only to be resolved in December 1988.
References

Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 558.|Potgieter, D.J. et al. (eds) (1970). Standard Encyclopaedia of Southern Africa, Cape Town: NASOU, v. 10, p. 176.