The Voortrekkers comprised two groups from the eastern frontier region of the Cape Colony, semi-nomadic pastoralists known as Trekboers, and established farmers and artisans known as Grensboere, or Border Farmers. Together these groups were later called Voortrekkers (Pioneers). While most settlers who lived in the western Cape (later known as the Cape Dutch) did not trek eastward, a small number did.
Historians have identified various factors that contributed to the migration of an estimated 12,000 Voortrekkers to the future Natal, Orange Free State and Transvaal regions. The primary motivations included discontent with the British rule: its Anglicisation policies, restrictive laws on slavery and its eventual abolition, arrangements to compensate former slave owners, and the perceived indifference of British authorities to border conflicts along the Cape Colony's eastern frontier. Many contemporary sources argue that Ordinance 50 (1828), which guaranteed equal legal rights to all free persons of colour, and prohibitions on inhumane treatment of workers, spurred the Boer migrations. However, some scholars argue that most Trekboers did not own slaves, unlike the more affluent Cape Dutch who did not migrate from the western Cape. The three republics subsequently founded by the Voortrekkers prohibited slavery, but enshrined racial separatism in their constitutions.
Other possible factors included the desire to escape from relentless border wars with the Xhosa along the eastern frontier of the Cape colony. The migrants also sought fertile farmland, as good land was becoming scarce within the colony's frontiers. The Great Trek also resulted from increasing population pressures, as Trekboer migrations eastward had come to a virtual stop for at least three decades, though some Trekboers did migrate beyond the Orange River prior to the Great Trek.
During the Great Trek the Voortrekkers engaged in conflict with the Zulu of Natal. The Zulu launched large-scale hostilities after a delegation under the Trek leader Piet Retief was massacred by their king, Dingane kaSenzangakhona on 6 February 1838.
Various interpretations of what exactly transpired exist, as only the missionary Francis Owen's written eye-witness account survived. Retief's written request for land contained veiled threats by referring to the Voortrekker's defeat of indigenous groups encountered along their journey. The Voortrekker demand for a written contract guaranteeing private property ownership was incompatible with the contemporaneous Zulu oral culture which prescribed that a chief could only temporarily dispense land, which was communally owned.
Most versions agree that the following happened: Dingane's authority extended over some of the land in which the Boers wanted to settle. As prerequisite to granting the Voortrekker request, Dingane demanded that the Voortrekkers return some cattle stolen by Sekonyela, a rival chief. After the Boers retrieved the cattle back, Dingane invited Retief to his residence at Umgungundlovu to finalise the treaty, having either planned the massacre in advance, or deciding to do so after Retief and his men arrived. Perhaps an earlier display of arms from horseback by Retief's men provoked the massacre. Dingane's reputed instruction to his warriors, "Bulalani abathakathi!" (Zulu for "kill the wizards") showed that he may have considered the Boers to wield evil supernatural powers. After murdering Piet Retief's delegation, the Zulu impis (battalions) immediately attacked Boer encampments in the Drakensberg foothills at what later was called Blaauwkrans and Weenen. In contrast to earlier conflicts with the Xhosa on the eastern Cape frontier, the Zulu killed the women and children along with the men, wiping out half of the Natal contingent of Voortrekkers.
On 6 April 1838 the Voortrekkers retaliated with a 347-strong punitive raid against the Zulu (later known as the Flight Commando), supported by new arrivals from the Orange Free State. They were roundly defeated by about 7,000 warriors at Ithaleni, southwest of uMgungundlovu. The well-known reluctance of Afrikaner leaders to submit to one another's leadership, which later so hindered sustained success in the Anglo-Boer wars, was largely to blame.
On 16 December 1838 a 470-strong force of Andries Pretorius confronted about 12,000 Zulu at prepared positions. The Boers reputedly suffered only 3 injuries without any fatalities, while the blood of 3,000 slain Zulu turned the river red with blood, so that the conflict afterwards became known as the Battle of Blood River. The Boers' guns offered them an obvious technological advantage over the Zulu's traditional weaponry of short stabbing spears, fighting sticks, and cattle-hide shields. The Boers attributed their victory to a vow they made to God before the battle: if victorious, they and future generations would commemorate the day as a Sabbath. Thus 16 December was celebrated by Boers as a public holiday, first called "Dingane's Day," later changed to the Day of the Vow. It is still a public holiday, but the name was changed to the Day of Reconciliation by the post-apartheid ANC government, in order to foster reconciliation between all South Africans. However, the Day of the Vow is still celebrated by Boers today.
After the defeat of the Zulu forces and the recovery of the treaty between Dingane and Retief from the latter's skeleton, the Voortrekkers proclaimed the Natalia Republic. This Boer state was annexed by British forces in 1843.
Due to the return of British rule, emphasis moved from occupying lands in Natal, east of the Drakensberg mountains, to the west of them and onto the high veld of the Transvaal and Orange Free State, which were unoccupied due to the devastation of the Mfecane.