JR Potts, AUS 173d AB and Dan Alex (Updated: 7/13/2016):
The US Navy sold two of the Brooklyn class cruisers to Argentina - USS Phoenix CL-46 and her sister ship, USS Boise CL-47 - on April 9th, 1951. The Argentine Navy (ARA) renamed the Phoenix the "ARA 17 de Octubre (C-4)" while the Boise was renamed "ARA Nueve de Julio (C-5)". After the Peron revolution, the ARA 17 de Octubre C-4 was again renamed to the "ARA General Belgrano (C-4)" after the father of the Argentine Navy. The Belgrano performed normal sea patrols and training duties in the South Atlantic, protecting the Argentine homeland and islands that Argentina felt was its undisputed territory.
For some time, Argentina and the United Kingdom were at odds with each other over the rightful ownership of the Falkland Islands, South Georgia and the South Sandwich Islands. On April 2nd, 1982, an undeclared war with Britain began through an Argentine invasion, and subsequent occupation, of the Falkland Islands and South Georgia which led to British civilians and military forces being taken prisoner. The initial invasion was considered lawful by Argentina as the islands in dispute have been under Argentine control since 1833. Conversely, Britain viewed the landings and the taking of prisoners as an invasion of legally-held British overseas sovereign territory.
Britain, not having a large naval force in the area, launched a naval task force to engage the Argentine Navy and Air Force to retake the islands. The Argentine Naval forces were outgunned numerically and outmoded by the then-current technology of the day when compared to the British. The UK had a modern first-world navy with nuclear submarines, large numbers of combat troops and a land airbase within striking distance. The ARA Navy was mostly of World War 2-era equipment though her aircraft were comparable to Britain's with full access to air-to-air and air-to-ship missiles. The Argentine Army had sufficient numbers but were not as well-trained as the UK Special Forces and general army personnel.
The cruiser Belgrano, though upgraded in some areas, was essentially the same ship as when commissioned in 1938, capable of speeds up to 32.5 knots. She retained her massive firepower with 15 x Mk 16 6-inch /47 cal (152mm) main guns able to fire a 130lb shell some 14.5 miles out. This was backed by 8 x 5-inch /54 cal (127 mm) guns and 40mm and 20mm anti-aircraft guns for close-in work. A British Sea Cat anti-aircraft missile system had been upgraded with two ready-to-fire missiles in 1967/68 though no test-firing had been done to this point. For supply and reconnaissance sorties, the Belgrano managed a pair of French Aerospatiale Alouette III helicopters. Her normal complement included 1,138 officers and sailors.