Not Trafalgar
That would be HMS Conqueror, who sank the ARA Belgrano (ex- USS Phoenix). Traditionally submarines returning from a war patrol fly a Jolly Roger, with different symbols dictating what sort of action took place. I've seen some good examples in the Imperial War Museum.
After the sinking of the Belgrano, which picked off one arm of a pincer movement which was being formed to attack the British fleet, the entire Argentinian Navy returned to port and didn't venture out for the rest of the conflict.
The head of the Navy, Admiral Anaya had been the prime mover in the Junta pressing for the invasion in the first place, but with the Navy now hiding in port, the very brave Argentinian pilots who were suffering great losses attacking the fleet, and the largely conscript Army on the islands had to ask themselves why they were taking all the pain. The fact that the Navy had abandoned them was a massive hit to morale.
That one attack by an ageing submarine, using WWII standard Torpedoes (the Captain decided not to risk using the new model he had on board) was almost certainly the most important action leading to the success of the Falklands campaign.
Biting the hand that feeds IT
