5 December 1934
The United South African National Party, also known as the United Party, was officially formed after a merger between the National Party, under the leadership of General J.B.M. Hertzog, and the South African Party under General J.C. Smuts. In 1933 the two parties put aside their differences and began to cooperate when they formed the coalition government.
References
Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau;| Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 429.