The invasion of the Falkland Islands had a very shocking effect on all those who were serving in the armed forces at the time.

We could not believe that in the post war era a relatively civilised country like Argentina could do such a thing.

Despite certain warnings seen in hindsight, there is no doubt that most of the military were completely taken by surprise.

As a relatively available captain in the infantry, with top secret security clearance, I was rung up within days of the invasion by a lieutenant colonel who I had met a few years earlier and seconded immediately into the Ministry of Defence main building in London.

I was to join three other staff officers (of major rank), to form a Falklands Islands intelligence cell. It was our job to examine all top secret intelligence sources and provide daily briefings for the chiefs of staff and Prime Minister on one hand and for the fleet on the other.

We had access to whatever we wanted from defence intelligence Section 3 (DI-3), which contained an army, navy and air force desk.

We also had secure direct radio telephone communications with the fleet and with the British embassy in Washington.

It was our job to understand everything that the other side was doing and why.

To this end we had a continuous flow of decoded and translated transcripts from GCHQ Cheltenham from sources as wide apart as embassy traffic, agents, ships and military units on the ground.

I have always been dismayed at what less informed journalist and writers have said about the sinking of the General Belgrano in what was the first offensive action of the war.

The film The Iron Lady, does present more facts about the decisions at the time, but still the impression given was that the torpedoing of the General Belgrano was a personal decision of the then Prime Minister, and was not particularly linked to other more substantial facts. The intelligence background was very comprehensive, and it is this that I can now present from one who was there.

At the outset of the occupation of the Falklands, the Argentinean General Menendez, stated that he would not leave until every last drop of his soldiers' blood was spilt and that he expected Argentina to occupy the islands for a thousand years.

In addition to this rhetoric, intelligence sources on the mainland of South America assessed that the junta was not willing to give up their claim to and occupation of the islands through diplomacy.

They were also doing all they could diplomatically through their foreign minister, Costa Mendez, to persuade allies such as the USA, that the islands were not really worth fighting over.

If Britain could be prevented from deploying a task force, then Argentina knew that its occupation would put them in a dominant and immovable diplomatic position.

There would be little chance of the inhabitants of the islands recovering their aspiration to be under the British crown. In this context, a judgement was made to send a task force as the only option to dislodge the occupying forces.

At times throughout the journey to the South Atlantic, Argentina could have signalled its desire to withdraw its occupying forces, and continue its diplomatic dialogue. However the mood in Buenos Aeries was a belligerent one, coupled with their belief that the British could not manage a successful invasion.

As the task force began assembling around the islands, radar showed that there were two Argentinean naval task forces, one outside the North Western edge of the exclusion zone and the other outside the South Western edge.

Both task forces were moving erratically, sometimes heading obliquely towards the exclusion zone, sometimes parallel to and sometimes away from the boundary. This was standard practice to confuse the enemy about eventual intentions.

At the time it was known that the General Belgrano task force carried Exocet missiles, for which the British ships had little defence.

It was assessed from directly intercepted radio traffic and mainland sources, that their intention was to form an attack from both North and South against the British naval forces and in particular to sink or put out of action the aircraft carriers.

To do this they only had to fire Exocet missiles a relatively short distance from outside the exclusion zone at two or more guard ships, which were at the time within range, and then to steam into the zone and attack the main British force from two directions.

The positions of the British ships had now reached a critical point around the islands, but there were two more factors which meant a decision had to be made soon.

One was that despite modern tracing radars and sonar, the British submarine, HMS Conqueror, was in danger of losing the Belgrano and its opportunity to attack, without itself being at risk.

The other factor was simply that, if the two Argentinean task forces were to attack as intelligence indicated, HMS Conqueror could only deal with one of them before they would then be in range of a number of other British ships. So the order was given.

It is not correct to say that because the General Belgrano was outside the exclusion zone, therefore it was not a threat. There was nothing in the rules of engagement, which ever stated that offensively intended Argentinean naval forces outside the zone were immune from engagement by the British.

The General Belgrano task force was an imminent threat to at least two in range British ships and certainly, having entered the zone, would have been a considerable threat to the rest of the British task force.

As one can also see, the direction of travel of the Belgrano task force was continuously variable and when the torpedoes struck, it may well have been facing away from the zone boundary, but this did not mean that the task force as such was returning to harbour, or had given up its offensive intentions.

The effect on the remainder of the Argentinean naval forces was to restrict the majority of them to their home ports for the remainder of the war.

This undoubtedly had the effect of saving British lives and shortening the entire war.

Whatever people might think about the war in general, there is no doubt that the decision to meet armed aggression with an armed response indirectly brought down a very unsavoury military junta, which had been responsible for oppressing its own people and causing many thousands to "disappear".

The year after the war, 1983, was a year of liberation for the Argentinean people and the return of democracy. It also heralded a significant religious revival in the country, which is going on to this day.

A few years ago, I had the privilege to speak at length to an Argentinean evangelist about the war and its aftermath.

He did say to me that their country had been wrong to invade and, that in his view and that of many, the British response had precipitated the downfall of the junta, which everyone in his country was extremely pleased about.

What I also learnt was that Argentina has always had strong ex British communities and their natural inclination was to be friends with us. The armed invasion of the Falklands was an aberration by a failing military dictatorship and as such had to be faced up to. Fortunately we had leaders at the time who were equipped to stand up to this bully regime.