In January 1879, Sir Henry Bartle Frere, a personal friend of Chelmsford, engineered a war against Cetshwayo, King of the Zulus, who at that time were an allied power by treaty with the British Crown. An expeditionary British imperial military force under Chelmsford's command subsequently entered the Zulu Kingdom uninvited, and was in consequence attacked on 22 January 1879 by a large Zulu army at Isandlwana, during which the Zulus overran and destroyed the central column of Chelmsford's separated forces. The engagement was an unexpected victory for the Zulus and one of the worst defeats of the British Army by native tribesmen in the history of the British Empire.