After the murder of Piet Retief and his 67 companions in February 1838, Zulu King Dingane ordered his forces to wipe out the remaining parties of Voortrekkers in Natal. Several unsuspecting Voortrekker laagers at the Bloukrans and Bushmans rivers (Weenen) were attacked by thousands of warriors in the early morning hours of 17 February 1838. Approximately 500 of Retief's followers were massacred, two-thirds of them women and children, and half of them Black. Tens of thousands of sheep, cattle and horses were driven away.

The Voortrekker leader Piet Uys fell with his men in a battle a month later while Hendrik Potgieter beat an ignominious retreat back to the highveld. Port Natal was razed to the ground, the surviving missionaries and traders escaping by ship.

It was at this point that Andries Pretorius reorganized the Voortrekker army and invaded Kwa-Zulu. The Battle of Blood River was fought, and the Voortrekkers defeated the Zulus. The Zulu nation never fully recovered from this defeat.

References:

1. Kruger, D.W. (ed) (1979). Geskiedenis van Suid-Afrika. NASOU, Goodwood, Cape Town, p. 213.
2. Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed. Academica, Pretoria, p. 165.
3. Potgieter, D.J. et al. (eds)(1970). Standard Encyclopaedia of Southern Africa. NASOU, Cape Town, v. 2, p. 381.
4. Wallis, F. (2000). Nuusdagboek: feite en fratse oor 1000 jaar, Kaapstad: Human & Rousseau.
5. Giliomee, H. and Mbenga, B. (2007). New History of South Africa. Tafelberg Publishers, Cape Town, pg 146