ON APRIL 2, 1982 Argentina invaded the Falkland Islands. Margaret Thatcher’s Government survived this humiliation by being able to announce to Parliament the next day that a substantial task force would soon set sail with the objective of “restoring British administration”.

It was widely assumed that the main value of this force was in strengthening the country’s bargaining position in the inevitable attempts to find a diplomatic solution to the emergency.

By the end of the month that assumption no longer looked so strong. On April 29 President Reagan came out in Britain’s support. Meanwhile the British had retaken South Georgia, a dependency of the Falklands, and the main carrier battle group was getting into position to enforce a 200-mile total exclusion zone which Britain had declared to prevent Argentina reinforcing its garrison on the Falklands. In addition it would be looking to persuade the Argentine Navy and Air Force out to fight in the hope of causing them severe damage. This was to prepare for an eventual landing on the Falklands.

To provoke the Argentinians into action, Rear Admiral Sandy Woodward, commanding the task force in the South Atlantic, decided to give the impression that the landings had already begun. This began early on May 1 with naval bombardments and air raids against military targets. According to Woodward, the aim was to “provoke a reaction that would allow me to conduct a major attrition exercise”, and in this he was successful. Aircraft flew from Argentine bases to confront the British and they suffered significant losses when challenged by the British Sea Harriers.

The Argentine Navy also prepared to fight the Royal Navy. During the evening of May 1 Admiral Jorge Allara, on board the Argentine aircraft carrier 25 de Mayo, was gearing up to mount an attack. His tracker aircraft had picked up signs of British warships 120 miles away and he was aware of the reports from the Falklands of attempted British landings. Admiral Juan Lombardo, Commander South Atlantic, had given him new rules of engagement, allowing for attacks on the British fleet without restriction. As the carrier group moved from the north a group led by the cruiser General Belgrano would move to the south of the exclusion zone to attack any British units there using the Exocet anti-ship missiles on the destroyers accompanying the cruiser, whose own 6in guns outranged the 4.5in guns used by the British.

The British plan was to attack the 25 de Mayo as soon as possible. The previous week the military had persuaded the War Cabinet to change the rules of engagement to allow a nuclear submarine to attack the carrier even outside the total exclusion zone, on the ground that the range of its aircraft would mean that it was a threat to British ships. Unfortunately for this plan, the carrier had avoided detection and was now closing in on the task force. Woodward knew this not only from intelligence on the Argentine orders to attack but also from one of his Sea Harriers that had come across the Argentine carrier group. At the same time another submarine, HMS Conqueror, had found the Belgrano group and was trailing it, but it had no permission to attack unless the group entered the exclusion zone (which intelligence had suggested it was going to do).

Woodward’s war diary noted that: “The Belgrano group is moving west [actually it was moving east], south of the Falkland Islands shadowed by Conqueror, and may wish to effect a pincer movement from that direction.” He suspected an attack by dawn on May 2 and so decided to concentrate his forces into defensive positions. Once he had got his ships together, Woodward began to feel a bit less exposed. Yet he was frustrated by the fact that he was not able to command the submarines directly and by Conqueror’s restricted rules of engagement. At issue was not just whether he could cope with an immediate Argentine offensive but the inhibitions that would be placed on his ability to move the British campaign along over the coming days. This frustration led him to order Conqueror to attack the Belgrano. As he anticipated, because of his lack of authority over Conqueror, the order was immediately rescinded.

As he also anticipated, this episode was brought to the attention of the Admiral Sir Terence Lewin, Chief of Defence Staff, who immediately sought political agreement for a change in the rules so that British ships and submarines could attack all Argentine naval ships, submarines and auxiliaries outside the total exclusion zone. At midday on May 2, as they gathered at Chequers to consider the course of the campaign, members of the War Cabinet were asked for a quick decision. The discussion took no more than 20 minutes. William Whitelaw later recalled it as “one of the simplest decisions that I personally found myself involved in”. Particularly compelling was the question of what the politicians would say if they had refused the military request when the Belgrano could have been sunk, and the cruiser then went on to sink a British carrier with hundreds of casualties. All were asked for their views before the Prime Minister gave her own but the issue was not controversial.

In the only official report, the decision was said to reflect “the latest intelligence about the movements and intentions of the Argentine fleet, and that of the new situation created by the military events of May 1”. Although the change of rules meant that the attack could take place outside the exclusion zone, the Foreign and Commonwealth Office later noted: “Ministers had also taken into account the fact that, in the present military situation, it was unlikely that any warships would need to be engaged very far from the exclusion zone.” Soon the new rules were being transmitted to the submarines.

By this time the situation had changed. Up to the early hours of May 2, the British had interpreted Argentine intentions correctly. Allara intended to use his Skyhawks to attack the British fleet and the Belgrano was part of a pincer. At this point, however, two developments changed the Argentine plans. First the wind dropped. Launching the Skyhawks required 40 knots. The wind continued to drop and the prospect now was of further delay and the aircraft taking off with reduced fuel and weapons loaded. Nonetheless, the group continued to close on the British.

The second event was the arrival of the Sea Harrier that had spotted the carrier group (convincing Woodward of the danger he faced). Although Allara’s initial instinct was to persevere, having been told by Lombardo that he did not know the position of the British carriers, so that the “free-ranging enemy still constitutes a strong threat”, he decided that there was little point in carrying on. The forecast gave him few reasons to expect better wind conditions. The task groups were ordered “back to their former positions”.

For most of the Sunday morning Woodward had no inkling of any change in Argentine plans but by 1100 GMT (0800 local time), with the light improving, the probability of an Argentine attack was diminishing. Woodward, however, was by no means confident. Only when it became dark did he intend to move west again to attack Argentine targets on the Falklands with naval gunfire. He knew that the Belgrano group was still being trailed by Conqueror; but not that it had reversed course two hours earlier. At 1400 GMT Conqueror reported the Belgrano’s new course to its headquarters at Northwood, but this information was not passed on to Woodward.

If it had been, Woodward’s reaction would have been the same as Northwood’s. A move in one direction could be followed by a move in the other. Concluding that an Argentine attack was not imminent was not the same as presuming that it had been postponed indefinitely. In terms of his original plan to flush out Argentine units, the best prospect he had now in terms of attrition was the Belgrano group. If it was not attacked that day then it would be returning to the fray the next time the Argentine Navy positioned itself for battle, by which time Conqueror could well have lost its contact. Woodward did not know that his wish had been granted, and approval for the new rules of engagement had already been agreed and transmitted to the submarines. He therefore again requested permission for Conqueror to attack the Belgrano.

Captain Chris Wreford-Brown on Conqueror had begun May 2 expecting to be following the Belgrano into the exclusion zone. He had been surprised by its change in course and remained uncertain as to which direction his quarry was intending to move for some time as it skirted the exclusion zone, perhaps aware of its vulnerability should it stray inside, and stayed some 18 miles to the south but still moving east. When Conqueror returned to periscope depth at 1400 GMT to receive and transmit signals, it reported on the change of direction to the west. Conqueror, however, had been suffering for some time with communication problems as a result of a damaged wireless mast (the crew had thought it might be necessary to withdraw to get the mast changed). Initially all Wreford-Brown understood was that the rules of engagement had changed: exactly how was unclear. He was aware of an order cancelling a previous order to attack (the original from Woodward — which he had not received) and that there appeared to be a new order to attack. Conqueror remained at depth until his instructions were clear. Now he was “absolutely certain”.

By the next transmission, at 1710, they were understood and this was reported back along with an intention to attack. Wreford-Brown chose the old Mk8 torpedo, in service since 1932, because as an impact weapon it had a better chance of penetrating the cruiser’s armour and anti-torpedo bilges. At 18.57 Conqueror attacked at a range of 1,400 yards. Two hits were observed, although three explosions were heard. By 19.30 the initial report of the attack had been transmitted. This gave the cruiser's position, course and speed when attacked, adding “successfully attacked Belgrano. Two hits with Mark Eights. Evaded to east”. The evasion was necessary because within a few minutes they were being attacked in turn with depth charges. From 20.52 to 21.03 more depth charges were heard.

On board the Belgrano some 200 men had been killed by the initial explosion, with fire spreading because doors and hatches had been left open. Another 850 took to life rafts as the cruiser began to sink. It sent out no signal of its own asking for help. The harsh weather and heavy sea conditions battered the crippled vessel and reduced the chances for survival of the crew as they abandoned ship. It took a day before the first survivors were picked up. In all 321 men of the Belgrano lost their lives.

Militarily this had exactly the effect required: the Argentine Navy did not venture out again. Politically it caused Britain damage, for the scale of the attack appeared disproportionate and the circumstances suggested that Britain was not following its own rules. After the war this led to claims that the attack had been ordered by the Prime Minister to make sure that there was no peace settlement. In fact the diplomatic fallout from the attack led the British to take peace initiatives much more seriously than before. The conspiracy theories were always wide of the mark. The Belgrano was sunk not because the British Government was confident in an inevitable military victory but because there was a serious risk of defeat.

• From The Official History of the Falklands Campaign, Volumes I and Il by Lawrence Freedman, published by Routledge Taylor & Francis on behalf of Whitehall History Publishing on June 28, priced £39.95 (Vol I) and £49.95 (Vol II). © Lawrence Freedman 2005