10 January 1879
Sir Bartle Frere was appointed British high commissioner to South Africa in 1879 to realise the policy of confederation (policy to bring the various British colonies, Boer republics and independent African groups under common control, with a view to implementing a policy of economic development). Sir Bartle Frere saw the self-reliant Zulu kingdom as a threat to this policy, a belief supported by Shepstone (Secretary for Native Affairs), who averred that the Zulu people had revived their military power under Cetshwayo, making them more of a threat to peace and prosperity in South Africa. On 11 December 1878, under the flimsy pretext of a few minor border incursions into Natal by Cetshwayo's followers, the Zulu were given an impossible ultimatum that they should disarm and Cetshwayo should forsake his sovereignty. The inevitable invasion of Zululand began after the ultimatum had expired on 10 January 1879. Instead of fragmenting the Zulu as Shepstone predicted, this made the Zulu rally to their king's cause. Many historians mark the 10 January 1879 as the beginning of the Anglo-Zulu War, while others claim that the 12 January 1879 (first attack) or the 22 January 1879 (first decisive battle of Isandlwana) marked the beginning of the war. Related links:SAHO feature on the Anglo-Zulu War.
References

Muller, C.F.J. (ed)(1981). Five Hundred years: a history of South Africa; 3rd rev. ed., Pretoria: Academica, p. 223.| Giliomee, H & Mbenga, B (2007). New History of South Africa. Tafelberg Publishers, Cape Town, pg 165.