alternatehistory.com

Prime Ministers of the Union of South Africa
1921-1924 Jan Smuts (South African)
def. 1921: J.B.M. Hertzog (National), F.H.P. Creswell (Labour)
1924-1929: J.B.M. Hertzog (National-Labour coalition) [1]
def. 1924: Jan Smuts (South African), F.H.P. Creswell (Labour)
1929-1933: J.B.M. Hertzog (National-Creswell Labour coalition)
def. 1929: Jan Smuts (United), F.H.P. Creswell (Creswell Labour), Walter Madeley (National Council Labour)[2]
1933-1938: J.B.M. Hertzog (National)
def 1933: Jan Smuts (United), Walter Madeley (Labour)
1938-1939: Jan Smuts (United) [3]
def. 1938: D.F. Malan ('Purified' National Party) [4], Walter Madeley (Labour)

Prime Ministers of the South African Federation
1939-1950: Jan Smuts (United)
def. 1943: Hubert Winthrop Young (Liberal) [5], Walter Madeley (Labour)
def. 1948: Hubert Winthrop Young (Liberal), John Christie (Labour)
1950-1951: J.G.N. Strauss (United)
1951-: Alan Paton (Liberal)
def. 1951: J.G.N. Strauss (United), John Christie (Labour)

[1] - Following the 1925 British Revolution, The Hertzog Government abolished the monarchy and instated a Republic, annexed South Rhodesia, Lesotho and Swaziland, and replaced ties to Britain with ties to Germany
[2] - Walter Madeley's faction split from Creswell's leadership over concerns about Madeley's support of Native labour union membership, as well as to protest the party's involvement in Government. This led to the National Party gaining an absolute Majority, and Creswell's faction folding into the Nationals.
[3] - Jan Smuts' United Party stormed to leadership with promises of restoring relations and trade with the Entente, as well as the promise to reform South Africa into a Federation. However this angered the Boers...
[4] - Triggering Malan's Hardliners to declare the Smuts Government illegitimate and launch the so-called Third Boer War, which resulted in the PNP being banned thereafter by Smuts, and the 'Loyalist' Nationals merging into the United Party. Malan's uprising was defeated, and with his newfound political capital enacted his Federation Plan
[5] - The Liberal Party broke off of the United Party in response to the Party's swing to the right after incorporating the National Party. They advocated for the extension of voting rights to all peoples of the Federation