Most Afrikaners were against South African participation in World War 1 on the side of the British. Therefore, when South Africa was asked to invade German South West Africa (SWA) in August 1914 there was opposition from the ranks of the newly formed National Party (NP), and even from some who were part of the South African government. At their August congress the opposed invasion, and on 15 August there was a republican demonstration in Lichtenburg. Besides these protest efforts, it was agreed that South West Africa should be invaded.

The economic depression after the war and dissatisfaction from Black South Africans and other extra-parliamentary groups made the SAP's rule more difficult. The main reason for black anger was Smuts' acceptance of the Stallard report that stated:

?It should be a recognised principle that natives ? men, women and children ? should only be permitted within municipal areas in so far and for as long as their presence is demanded by the wants of the white population?The masterless native in urban areas is a source of danger and a cause of degradation of both black and white?If the native is to be regarded as a permanent element in municipal areas?there can be no justification for basing his exclusion from the franchise on the simple ground of colour.? (This report later led to the passing of the Natives (Urban Areas) Act no 21 of 1923).

The Afrikaner opposition to WW1 proved to strengthen the, particularly after the death of General De la Rey (Afrikaners blamed Smuts and Botha). The death of General Louis Botha in 1919 pushed away more of the SAP supporters, and by the end of the Great War many of the SAP?s supporters had left the party and joined the.

In the 1920 elections it became clear that the SAP would need the?s cooperation to form a combined cabinet, in order to maintain political stability. Members of both parties met at Robertson on 26 and 27 May 1920, and made a potential agreement. On 22 September the two parties met again, but they could not finalize an agreement. The main point of disagreement concerned South Africa?s relationship with Britain -  Hertzog wanted independence, while Smuts was happy with the situation as it was.

The Rand Rebellion of 1922 further strengthened the?s popularity, as it led to cooperation between the and the Labour Party (LP). The Rebellion was the result of severe labour unrest that had been simmering for some time. Both parties wanted to protect White labour, and decided to make a pact in April 1923 that would ensure that they would not oppose each other in the elections, and would support each other?s candidates in certain areas. This Pact resulted in the defeat of the SAP in the 27 June 1924 general elections. Afrikaans then became an official language and the country got a new flag.