Jack,
I never suggested that the Americans had warning, the Brits had warning some 4 years previously when many C130's, with payloads, under a Labour government, were dispatched to Ascension with the facility to continue to Stanley.
Then before the invasion of the Falklands the Argies had landed on South Georgia, had a gunfight with the few marines present, then raised the Argie flag.
The Brits didn't need a 'tip off' from the Americans, they'd had enough tip offs themselves but there remained Maggie, counting the pennies in her budget, yes let us retire HMS Endurannce, pull the marines out of the area, and we'll save two shillings and sixpence.
It was an accident waiting to happen but didn't Maggie milk it for all it was worth, at every re-election campaign thereafter, her (jeopardising) saving of the Falklands kept her in power for years thereafter!