28 May 1901
Allegedly Zulu Paramount Chief Dinuzulu (Dinizulu) refused British instructions to take up arms against the Boer republics during the Second Anglo-Boer War. Col. H.B. Bottomley of the Imperial White Horse warned the king that, under martial law, he was obliged to follow orders or face banishment. In March 1901, British military law had been imposed on the region and the Zulu people were armed by the British. There is controversy about the date of 28 May, as another source gives it as 31 May. The accuracy of this event can be debated as another tradition states that a Zulu force of 6,000 men was assembled by Dinuzulu and Bottomley at oSuthu and advanced against Boers who had fled with their cattle to the Dleke hill, about forty kilometres from Nongoma. The military operation from 4 - 9 April 1901 ended in a Boer defeat. Thousands of cattle, guns and ammunition left on the wagons by the fleeing Boers were captured. Click here to read more about the role of the Zulus in Anglo-Boer War 2.
References

Cloete, P.G. ( 2000). The Anglo-Boer War: a chronology, Pretoria: Lapa.|cyclopaedia.net,"
Dinuzulu",From: ,">cyclopaedia.net,[Online],Available at: cyclopaedia.net ,[Accessed on: 26 May 2014]