The Union of South Africa is the historic predecessor to the modern-day Republic of South Africa. It was formed on 31 May 1910 when the British Cape Colony and the Natal Colony unified with the defeated Boer republics of the South African Republic and the Orange Free State. The provinces of the union were called, respectively, as the Cape Province, the Natal Province, the Transvaal Province, and the Orange Free State Province. Following World War I, the former German colony of German South-West Africa was put under administration of the Union of South Africa as a League of Nations mandate and was treated as if it were another province.