Saul David is the author of Zulu: The Heroism and Tragedy of the Zulu War of 1879. For more information on this title, and to purchase your copy online, click here.

Rorke's Drift, Natal. Two in the afternoon. Lieutenant John Chard is writing a letter in his tent, sheltering from the glare of the fierce African sun, when a sentry reports the approach of two riders from the Zulu side of the river. Once across they blurt out the news: the nearby camp at Isandlwana has been attacked and taken by thousands of Zulus; scarcely a man has escaped. The Zulus, moreover, were "advancing on Rorke's Drift".

As an appalled Chard tries to make sense of the news, a message arrives from Lieutenant Gonville Bromhead, commanding the troops at the nearby mission house, requesting Chard's presence.

He gallops over to find the mission a hive of activity. Tents are being struck and every available man is busy barricading and loopholing the mission's two stone buildings – in use as a store and a hospital – and connecting them with a wall of wagons and mealie sacks. Bromhead shows Chard a pencil-written message he has just received from an officer at Isandlwana. The camp has fallen, it says, and the mission at Rorke's Drift is to be fortified and held "at all costs".

Having discussed the message with Bromhead, Chard issues new orders "to strike the camp and make ready to go". Clearly neither officer thinks much of the garrison's chances of holding out. Fortunately a calmer head is close at hand: Assistant-Commissary James Dalton, an experienced former NCO. Dalton is in charge of the stores at Rorke's Drift and can see no sense in flight. If they abandon the mission, he tells Chard, they will be overtaken by fleet-footed Zulus and "every man was certain to be killed".

Brought to his senses, Chard orders the work on the defences to continue. It is supervised by Dalton "whose energy, intelligence and gallantry", recalls Chard, "were of the greatest service to us".

At 4.30 p.m., a look-out reports a huge Zulu column over the brow of the hill. The spellbound redcoats are almost relieved when the Zulu vanguard finally appears: 600 glistening warriors with animal tail kilts and ostrich-plume headdresses. Carrying rifles, rawhide shields, knobkerries and short stabbing spears, they appear at the run from the back of the hill, drive in the thin line of British skirmishers and make straight for the south wall. "Here they come!" shouts one defender as the mission explodes in a crash of volley fire. The Battle of Rorke's Drift has begun. 

For 12 hours the tiny British garrison of 140 men – many of them sick and wounded – fought to repel the repeated attacks of up to 4,000 Zulu warriors. This heroic defence was rewarded with no fewer than 11 Victoria Crosses – the most for a single action – and later immortalized by the film Zulu (1964). Yet few remember it was fought on the same day the British Army suffered its most humiliating defeat at nearby Isandlwana. Why? Because it suited those responsible for the disaster to exaggerate the importance of Rorke's Drift in the hope of reducing the impact of Isandlwana.

The true story of 22 January 1879 - the Empire's longest day - is one of unprovoked slaughter, of heroes being ignored and of the guilty being protected. And the responsibility for this went right to the top of the British government – to Queen Victoria herself.

Like so many imperial conflicts of the period, the Zulu War was not initiated from London. Instead, Benjamin Disraeli's government - preoccupied with the Russian threat to Constantinople and Afghanistan - made every effort to avoid a fight. "We cannot now have a Zulu war, in addition to other greater and too possible troubles", wrote Sir Michael Hicks Beach, the colonial secretary, in November 1878.

The man to whom this letter was addressed - Sir Bartle Frere - had others ideas, however. Frere had been sent out to Cape Town as high commissioner with the specific task of grouping South Africa's hotch-potch of British colonies, Boer republics and independent black states into a Confederation of South Africa. But he quickly realised that the region could not be unified under British rule until the powerful Zulu kingdom - with its standing army of 40,000 disciplined warriors - had been suppressed.

So he exaggerated the threat posed by the Zulus to the British, and, when the home government refused to sanction war, took matters into his own hands in December 1878 by presenting the Zulu king, Cetshwayo, with an unacceptable ultimatum. This required, among other things, the disbandment of the Zulu Army, and war was the inevitable result.

Such unilateral action by an imperial pro-consul was not unusual during the Victorian period. So great were the distances involved, and so slow the methods of communication, that British governors often took it upon themselves to start wars and annex provinces. Indeed, Lord Lytton, the viceroy of India, was about to invade Afghanistan without any reference to London at all. But the Zulu conflict was unique in that it was to be the last pre-emptive war launched by the British, prior to the recent campaign in Iraq.

The war began on 11 January 1879, when the 5,000-strong main British column invaded Zululand at Rorke's Drift. It was commanded by the ambitious Lord Chelmsford, a favourite of the Queen, who had little respect for the fighting qualities of the Zulu. "If I am called upon to conduct operations against them," he wrote in July 1878, "I shall strive to be in a position to show them how hopelessly inferior they are to us in fighting power, altho' numerically stronger."

This dangerous mixture of self-confidence and contempt for their foes infected the whole British force. But their misjudgement came to rebound on them badly.

By 20 January - hampered by minor skirmishes and poor tracks - Chelmsford's column had only advanced 11 miles to the rocky lower slopes of a distinctive, sphinx-like hill called Isandlwana. There it set up camp. But at 4am on 22 January, Chelmsford made the first of a series of blunders by taking two-thirds of his force off to pursue what he believed was the main Zulu army.

He was convinced that the Zulus were gathering to the south-east, and so failed to reconnoitre adequately the broken ground to the north-east. There, lying in wait just five miles from the exposed camp at Isandlwana, were 20,000 Zulu warriors.

At around 8am, mounted patrols reported large numbers of Zulus on the high ground to the left of the camp. Colonel Pulleine, in command at Isandlwana, dashed off a quick note to Chelmsford, reading: "Report just come in that the Zulus are advancing in force from Left front of Camp." Chelmsford received it shortly after 9.30am, but ignored it. He would not be deflected from his original plan.

At 11am, by which time the 1,300 men remaining in the camp had been swelled by 450 reinforcements, mounted scouts stumbled upon the concealed Zulu impi. Realising they had been spotted, the Zulus rose as one and began their attack, using their traditional tactic of encirclement known as the izimpondo zankomo ("horns of the buffalo").

An hour later, as the hard-pressed British defenders fought for their lives, a portion of Chelmsford’s force at Mangeni Falls received word that the camp was in danger of being overrun. On his own initiative a Colonel Harness gave orders for his small force of artillery and infantry to return to camp. But it had only progressed half a mile when a staff officer rode up with express orders from Chelmsford to resume its original march because the message was a false alarm. The last chance to save the camp had been thrown away.

By 3pm, despite severe losses, the Zulus had captured the camp. The culmination of Chelmsford's incompetence was a blood-soaked field littered with thousands of corpses. Of the original 1,750 defenders - 1,000 British and 750 black auxiliaries - 1,350 had been killed.

Word of the disaster reached Britain on 11 February 1879. The Victorian public was dumbstruck by the news that "spear-wielding savages" had defeated a well-equipped British force. The hunt was on for a scapegoat, and Chelmsford was the obvious candidate. But he had powerful supporters.

On 12 March 1879 Disraeli told Queen Victoria that his "whole Cabinet had wanted to yield to the clamours of the Press, & Clubs, for the recall of Ld. Chelmsford". He had, however, "after great difficulty carried the day". Disraeli was protecting Chelmsford not because he believed him to be blameless for Isandlwana, but because he was under intense pressure to do so from the Queen.

Meanwhile Lord Chelmsford was urgently burying all the evidence that could be used against him. He propagated the myth that a shortage of ammunition led to defeat at Isandlwana. He ensured that potential witnesses to his errors were unable to speak out. Even more significantly, he tried to push blame for the defeat onto Colonel Durnford, now dead, claiming that Durnford had disobeyed orders to defend the camp.

The truth is that no orders were ever given to Durnford to take command.

And Chelmsford had another weapon to use against his critics - that of Rorke's Drift. Though undeniably heroic, the importance of the defence of Rorke's Drift was grossly exaggerated by both the generals and politicians of the period, to diminish the impact of Isandlwana. "We must not forget," Disraeli told the House of Lords on 13 February, "the exhibition of heroic valour by those who have been spared."

Within days of Rorke's Drift, Chelmsford was urging the speedy completion of the official report because he was "anxious to send that gleam of sunshine home as soon as possible". When it finally arrived, he added two names to the six recommended VCs - those of lieutenants Chard and Bromhead.

Many of their fellow officers were amazed. One wrote: "Bromhead is a great favourite in his regiment and a capital fellow at everything except soldiering ... He had to be reported confidentially as hopeless." Another described Chard as "a most useless officer, fit for nothing".

In truth, the real hero of Rorke's Drift was Commissary Dalton. It was Dalton who persuaded Chard and Bromhead to remain at Rorke's Drift when their first instinct was to abandon the post, and it was Dalton who organised and inspired the defence. But Dalton, an ex-NCO, came from what was considered the wrong background, and was ignored for almost a year. He was eventually awarded a VC after intensive lobbying by the press - but not until January 1880, by which time the celebrations had died down.

Back in England meanwhile - with the Zulu War no nearer to being won - the cries for Chelmsford's recall intensified. On 23 May, realising that his political future was on the line, Disraeli told the queen that his government was replacing Chelmsford with Wolseley. She replied frostily: "I will not withhold my sanction though I cannot approve it." It was one of the few serious breaches she and Disraeli had during their political relationship.

In early September, shortly after his return from South Africa, Lord Chelmsford was given an audience with the Queen. She recorded the conversation in her journal:

"Ld. Chelmsford said no doubt poor Col. Durnford had disobeyed orders, in leaving the camp as he did... Ld. Chelmsford knew nothing, Col. Durnford never having sent any message to say he was in danger... This much is clear to me: viz. that it was not his fault, but that of others, that this surprise at Sandlwana took place... I told Ld. Chelmsford he had been blamed by many, and even by the Government, for commencing the war without sufficient cause. He replied that he believed it to have been quite inevitable; that if we had not made war when we did, we should have been attacked and possibly overpowered."

Most of what Chelmsford told the Queen was a pack of lies. Durnford, as we have seen, did not disobey orders. And Chelmsford ignored at least two warnings to the effect the camp "was in danger". In addition, the war was not one of self-defence but of conquest. Queen Victoria, however, would not see the truth.

The British captured King Cetshwayo in August 1879, and the war, to all intents and purposes, was over. But few emerged on the British side with any credit, nor did ordinary Zulus benefit. Cetshwayo was exiled, Zululand was broken up and eventually annexed. Frere never achieved his ambition to confederate South Africa. That would have to wait until the aftermath of an even bloodier conflict, that of the Boer War.

Disraeli lost the 1880 election and died the following year. James Dalton died in 1887, a broken man. Many of the lower-rank VC winners from Rorke's Drift were also forgotten when the media circus moved on.

But one man prospered - Lord Chelmsford. The Queen showered honours on him, promoting him to full general, awarding him the Gold Stick at Court and appointing him lieutenant of the Tower of London. He died in 1905, at the age of 78, playing billiards at his club.

Saul David is the author of Zulu: The Heroism and Tragedy of the Zulu War of 1879 (Penguin, £8.99). He was the programme consultant for the recent Timewatch documentary 'Zulu -The True Story'. His latest book, Victoria's Wars: The Rise of Empire (Viking, £25), is published in May 2006. For more information on these titles, or to purchase your copy online, click here.