first Prime Minister of the Union of South Africa (1910–19). One of the most successful Boer leaders in the Boer War, Botha became commander-in-chief in 1900 and waged guerrilla warfare against the British forces. As Transvaal’s first Prime Minister he played a leading role in the National Convention (1908–09), which was responsible for drafting the constitution for the Union of South Africa; he became its Prime Minister a year later. Botha supported the Allies in World War I, gaining recognition for his annexation of German South West Africa in 1915.