The rules of engagement were under "green card." This is the same set of rules as used in Northern Ireland at the same time.

The rules of engagement are decreed by the government.
The task force was dispatched to retake the Falklands by force of arms, as the Argentinians had taken them.
The Conqueror sought and was granted permission from the government to engage.
The rules of engagement were to sink her.
Belgrano was under orders to sink a British carrier.
That is all established fact.