The Belgrano was attacked by HMS Conqueror at about 19:00 UK time on the 2nd May 1982, as she was withdrawing from an attempted sortie against the British Task Force. At the time of the attack, she was outside the Total Exclusion Zone (TEZ). The TEZ was a 200- nautical mile ring around the Falklands within which the British reserved the right to attack any ship or aircraft sighted without warning. In theory, by attacking the Belgrano outside the TEZ, the British were committing a war crime. However, this ignores the fact that the Belgrano was a warship of a belligerent power. The TEZ gave the British the ability to take action against Argentinian and neutral civilian ships that were shipping supplies to the Falklands, or were spying on the actions of the British Task Force. Warships could be, in theory, attacked anywhere.

Confusing the issue, though, was the Maritime Exclusion Zone (MEZ) which preceded the TEZ, as well as the Rules of Engagement issued to British submarines. The Maritime Exclusion Zone was declared on the 12th April. It covered the same 200-mile circle that the TEZ did, but, rather than applying to all ships, applied only to Argentine naval vessels and auxiliaries. Outside the circle, they could only be attacked in self-defence. Under this, sinking the Belgrano without warning would be a war crime. However, the MEZ was superseded by the TEZ from the 30th April. The British intentions had also been clarified in a statement on the 23rd April, which stated that Argentinian warships moving to threaten the Task Force could be attacked outside the MEZ. While Belgrano was not, at the time of the attack, in a position to threaten the Task Force, her actions were ambiguous, and, had her commander chosen to, she could have. The Rules of Engagement (ROE) Conqueror was operating under had changed several times during the conflict in the run-up to the attack on Belgrano. Initially, British submarines were only able to engage in self-defence. From the 12th, they could attack identified Argentinian warships, auxiliaries and submarines inside the exclusion zone. Outside it, they could only fire in self-defence, and only after being attacked. On the 20th April, the submarines were given permission to engage Argentinian vessels showing 'hostile intent' inside the 200 mile 'Defence Zone' Argentina had drawn around South Georgia. On the 27th, they were allowed to attack any diesel-electric submarine detected, in or outside the exclusion zone. On the 30th, they were allowed to attack the Argentinian carrier 25 de Mayo outside the exclusion zone, but only after British forces as a whole were attacked by any Argentinian unit. This attack duly happened on the 1st May, with air attacks being launched against the Task Force (albeit to little avail). These attacks, combined with movements of the 25 de Mayo and Belgrano into threatening positions, led to a loosening of the ROE to allow attacks on any Argentinian warships, inside or outside the TEZ, at 11:45 AM on the 2nd. Conqueror's attack on Belgrano would have been in breach of the earlier ROEs, but under this set, it was perfectly fine.

From the Argentinian perspective, the attack was perfectly legal. Conqueror was operating within Argentina's own exclusion zone. This covered areas within 200 miles of the Argentinian mainland, the Falklands, and the various Dependencies of both. British ships and submarines in this area were legitimate targets, and the Argentinians accepted the risk of attack. Rear Admiral Allara, commanding the larger Argentinian force that Belgrano was a part of, would later state that

After that message of 23 April, the entire South Atlantic was an operational theatre for both sides. We, as professionals, said it was just too bad that we lost the Belgrano.

The Belgrano's captain, Captain Hector Bonzo, had similar sentiments:

I realized from the outset that the 200-mile limit had nothing to do with the mission I had to accomplish. The limit did not exclude danger or risks; it was all the same in or out. I would like to be quite precise that, as far as I was concerned, the 200-mile limit was valid until 1 May, that is while diplomatic negotiations were taking place and/or until a real act of war took place, and that had happened on 1 May.

While this was the view of the Argentinian Navy, it was easy for the Argentinian government to portray Britain as being in breach of its own rules. The declaration of the TEZ strongly suggested that combat would only occur within it. Attacking a target outside of it seemed, on the surface, to be a clear breach of its intentions. This made the sinking of the Belgrano an easy target for complaints by the Argentinian government, and by anti-war groups in Britain. It also led to a reduction in support for the British cause from other governments.