This thesis considers the relationship between Germany’s South-West African colony and its British South African counterparts (the Cape Colony, Natal, Rhodesia and, after the second Anglo-Boer War, the Orange River Colony, and the Transvaal) between 1883 and 1915. The chapters consider the complex and fraught relationship, including the British Government’s surprise and the Cape Government’s dismay following Germany’s establishment of the colony: the German public’s pro-Boer stance juxtaposed against the German Government’s refusal to intervene during the second Anglo-Boer War; the Cape Government’s dilemmas over whether to aid German South-West Africa (GSWA) during Germany’s quasi-genocidal campaigns against the Herero and the Nama; efforts to cooperate with German South-West Africa despite labour competition during the period of the unification of South Africa; and the period after 1910, when the diplomatic relationship became an affair of the Union of South Africa, which simultaneously pursued protectionist policy for South African trade, and bilateral cooperation concerning the diamond industry, as well as security along the border between 1911 and 1914. Finally, I consider the impact of the outbreak of the First World War, which saw Germany and GSWA offer support for an Afrikaner Rebellion to draw Britain’s attention away Europe and install a friendly government in South Africa, while also offering the Union an opportunity to conquer GSWA as part of its sub-imperial ambitions. Among the enduring themes are the interplay between political, economic and military developments, including border disputes, illicit trade, labour competition, and armed incursions led by non-state actors. In conclusion, I argue that as the idea of a South African federation progressed, it was driven in part by geopolitical factors and the desire to counter German imperialism. The British Government endorsed a South African union in part to create a South Africa strong enough to fend off German geopolitical threats. / Thesis (PhD (History))--University of Pretoria, 2021. / Historical and Heritage Studies / PhD (History) / Unrestricted