Isandlwana (it was a large irregular-shaped height) lay about ten miles out from Rorke's Drift. Lord Chelmsford camped there because heavy rains required that the road they were following be repaired. The area around Isandlwana offered a plentiful supply of drinking water and driftwood for fires, and appeared to offer the field of fire that Chelmsford felt was necessary. He did not, however, order the men to entrench, since he believed that superior British weapons afforded him an insuperable advantage over the Zulus.
The position was not as impregnable as Lord Chelmsford imagined, however. The large area of open ground was in fact pitted with numerous gullies that were obscured by brush and through which fairly large groups of Zulus could pass undetected by British sentries. He also elected not to circle (laager) his wagons, even though it would have made his position more secure, because doing so was a complicated business, and some of the wagons were to be sent back shortly to Rorke's Drift to resupply. He did set out pickets and a mounted vidette, although these early warning systems were posted much closer to camp than officers in Chelmsford's Native Contingent advised. Chelmsford also sent a mounted patrol out towards the east, which reported no sign of Zulus, although reports were filtering in of Zulu activity to the south (the direction from which it would be least expected).
On the morning of 22 January 1879, Lord Chelmsford himself set off with 2,500 men to do a reconnoiter of the area where Zulus had supposedly been sighted in force. He also took four guns with his column. This left a force of around 2,000 men to defend the camp at Isandlwana. Of these, 1,168 were Natal volunteer force troops or Natal Native Contingent.
Shortly before noon on 22 January 1879, a British patrol crested a rise and found themselves in the midst of a Zulu army. This army was composed of at least 20,000 Zulu warriors. The British fired one volley and quickly withdrew, sending messengers in the meantime to warn other patrols, Chelmsford, and the camp at Isandlwana.
As the main Zulu column approached the camp (around 12:30), artillery fire greeted them. Apparently, this fire began while the Zulu were still well out of range, so the fire did little damage to the Zulu. When they got closer, the Zulu closest to the guns threw themselves down when they saw the gunners step away from their guns (an indication the gun was about to be fired).
The Zulu advanced in 20 or so well-disciplined rows, the chest and left horn appearing to observers to be maintaining somewhat better order than the right horn. The British regulars at this point, however, were well-ordered, and their continuous and very accurate fire kept the Zulu attack at bay at a range of around 400 yards (the Martini-Henry's optimum firing distance). Some Zulus were armed with the same weapon, but their fire did much less damage (reports said that they generally fired high and did little damage to the British).
The British maintained discipline and a steady, controlled fire for approximately one hour, at which time their line disintegrated and all hell broke loose. It has been suggested that the British had trouble getting ammunition boxes open and in transporting ammunition to the firing line, but the ammunition boxes were secured with one screw that could be dislodged if necessary with a gun butt. Testimony of several survivors suggests that bandsmen and wagon drivers were set to work carrying ammunition to the line.
Two other problems that may have contributed to the British collapse have been suggested. The first suggests that continuous firing of the Martini-Henry often caused problems with the weapon's lever action, and a cartridge in the chamber would have to be removed by hand before the weapon could be fired again. Testimony mentioning this difficulty, however, concerns the Natal volunteers and the Natal Native Contingent, not British regulars.
The second problem suggested is smoke. Continuous firing of artillery and black powder firearms raising a choking, sight-obscuring cloud of black smoke that makes it hard to breathe and hard to see your targets. In the time shortly before the British collapse, the regulars were mostly firing blind, hoping to hit a Zulu in the smoke and confusion.
Once a withdrawal from camp was apparently ordered by the senior officer, the British firing line disintegrated and the Zulu rushed forward to follow the retreating men. Testimony from Zulu attackers and British survivors suggests that the men in the firing line were stretched too thin, and that there were perceptible gaps between units in the firing line.
An often overlooked reason for the British collapse, of course, was the bravery and determination of the attacking Zulus. As George Pickett said when asked why the advance named for him had failed, "I think the Union Army had something to do with it." British firepower initially stalled the Zulu attack-- they had never before faced such weapons. But attackers were reformed and the attack pressed forward, in spite of terrible losses in the front ranks of the attackers. There is no definitive count of Zulu casualties at Isandlwana, but best estimates place Zulu dead on the field at 1,000 or slightly higher. Many other Zulu warriors were wounded, many badly so-- and with medical care available to battle casualties rudimentary at best, it seems highly likely that many additional warriors died later due to wounds received at Isandlwana.
The majority of British casualties were caused by the Ikwa-- the Zulu stabbing spear, with a much lesser number caused by the throwing spear. Some British casualties were caused by small arms fire, partly from older weapons Zulu had acquired prior to the war, the rest from Martini-Henry rifles picked up from soldiers killed earlier that day. Since the Zulu were not trained in the use of those weapons, however, they were not as deadly as they were in the hands of British regulars. Soldiers who were not killed outright in the Zulu attack were killed by being disemboweled-- the Zulu believed that doing so was necessary to release the spirit of a warrior (friend or foe) into the afterlife. No wounded soldiers survived-- all were killed by the Zulu. There was also some mutilation of British corpses beyond the ritual disemboweling-- some soldiers were also beheaded, along with other, even more unsettling desecrations. A press report after the battle noted that two drummer boys had been mutilated while still alive, although experts disagree on whether the boys were dead or alive when the mutilations took place (the report did bring to an end the practice of taking drummer boys in the field with their regiments). Looting of corpses also occurred, in part because Zulu religious beliefs demanded that a warrior take an article of clothing from a man he had killed.
A majority of Zulu casualties were caused by small arms fire, specifically fire from the Martini-Henry rifle. Accounts of the battle in later years by Zulu survivors mention, however, that when ammunition for rifles ran out, soldiers used pistols, bayonets, and the butts of their rifles as clubs. Some soldiers were killed while attempting to flee towards the Buffalo River, but a majority were killed in groups in and around the camp at Isandlwana.