Complex sentence: The Boers (Afrikaner Farmers), had established the Transvaal as their promised land, after their Great Trek out of the Cape colony, a trek whose purpose was to get away from the British rule.
Split into simpler sentences: The Boers (Afrikaner Farmers), had established the Transvaal as their promised land, after their Great Trek out of the Cape colony. A trek which's purpose was to get away from the British rule.