The movement northwards in the 1830s by Boers to escape from British administration in the Cape Colony. From 1835 onwards parties of Voortrekkers reached Natal, where in 1837 Zulu resistance provoked them to kill some 3,000 Zulus at the Battle of Blood River in revenge for the death of their leader, Piet Retief. Natal became a British colony in 1843 and migration continued northwards into the Orange River country and the Transvaal.