Colt is referring to the events of the 8th Xhosa War, which ran from 1850 to 1853 in South Africa. The British Army engaged with the Xhosa in support of Boer settlers who had moved to the area. The ensuing wars lasted, on and off, for a hundred years, from 1779 to 1879.
During the 8th Xhosa War, Xhosa forces under the leadership of Maqoma gathered at Water Kloof (Watercloof Ridge), a forested area which was easy to defend. Over the next two years, they inflicted heavy losses on the British troops under Sir Harry Smith. In 1853, the Royal Navy ship HMS Birkenhead was wrecked while bringing reinforcements, mostly from 73rd Regiment (Edward's cousin's regiment), which lost a great number of men.