After several frontier campaigns against the Xhosa he took part in the Great Trek, and became commandant-general of the Boers after Piet Retief’s murder (1838). He defeated the Zulu at the Blood River (1838). In 1847 he organized protests against the British annexation of the land between the Orange and Vaal, but in 1848 he was defeated at Boomplaats by Sir Harry Smith, when the Orange River Sovereignty was established. In 1852 he was instrumental in negotiating the Sand River Convention, which recognized the land beyond the Vaal as the South African Republic.