The South African Army was formed in 1912. It had five regular mounted regiments and a small artillery section. Conscription was also introduced in 1912 and half of the European males aged 16 to 25 were drafted by lots into the Active Citizen Force (ACF).

On the outbreak of the First World War, General Luis Botha, the president of South Africa, immediately offered to send troops to invade German South-West Africa. Afrikaner opposition to this move provoked a major Boer Revolt. This was defeated by the middle of 1915 but in the elections of that year, Botha's National Party, only narrowly held on to power.

Led by General Jan Smuts, over 146,000 men served in South African units during the war. This included 43,000 in German South-West Africa and 30,000 on the Western Front. It is estimated that about 3,000 South Africans joined the Royal Flying Corps. Total South African casualties during the war reached 18,600 with over 6,600 being killed.