many nations, even Britain's allies treated it as a possible war crime.

No they did not.

Musket and Jim,
The Independent, 3rd April 2012
Rear-Admiral Walter Allara, in command of the Belgrano battlegroup, said: "The entire South Atlantic was an operational theatre for both sides. We, as professionals, felt it was just too bad that we lost the Belgrano." Admiral Enrique Molina Pico, head of the Argentinian Navy in the 1990s, said the cruiser was part of an operation that posed a real threat to the British task force and that she was holding off from carrying out an attack for purely tactical reasons.