Question:

Why did Argentina seize South Georgia immediately prior to the Falklands invasion?

Isabella: 3 days ago

What purpose was served by Argentina's occupation of South Georgia immediately prior to the invasion of the Falklands? The famously elaborate operation, involving marines masquerading as scrap metal merchants and obscure diplomatic slights would appear to have achieved nothing of value. A miniature side show compared to the main Falklands war. What did Buenos Aires intend it to achieve?

If it was meant as a test of British resolve it surely made no sense to launch the full Falklands invasion almost in parallel and before evaluating the British response. If it was intended as a diversionary tactic it turned out to be anything but. If Argentina intended it to be strategically vital to defending the Falklands they were deluded since the British easily retook it some weeks later.

Why bother with South Georgia as a preliminary campaign? If, as Argentina intended and hoped, the Falklands occupation had been accepted by the British and the rest of the world as a fait accompli then South Georgia would quietly have been included in any eventual settlement.

Answer:
Eli: 3 days ago

The main reason as I understand it was to extend the Argentine territorial waters and thus create a stronger claim to ownership of the Falklands. It would also seriously deplete the British forces in the Falklands to oppose the invasion (though that was of course not guaranteed).
Of course there's also the potential wealth of oil, gas, and minerals in and around the islands, but those were not as important back then as they would be today (the cost of extracting them was relatively much higher because of the lower prices of oil and minerals in the 1980s, making extraction at the time not economical).

South Georgia was invaded in early March, on the 19th the British noticed an Argentine flag flying over the islands, well away from their own scientific station there. The Argentinians had landed armed forces disguised as scrap metal dealers on the islands.
The next day HMS Endurance, embarking part of the Royal Marine detachment to the Falklands, sailed from Port Stanley to confront the Argentine landing force at South Georgia. On the 24th Endurance reached South Georgia, but was ordered to reinforce the British outpost there rather than sail around the island to the Argentine camp, possibly because of uncertainty about Argentine strength in the area.
This was fortuitious, as that same day the Argentinians landed another roughly 100 troops on the island (Endurance carried only 20 Marines and was itself only lightly armed, more a police vessel than a warship).

Directly following this, the Argentine junta found itself pretty much forced by general Anaya (who had orchestrated the South Georgia affair seemingly on his own) to bring forward the invasion plans for the Falklands. The decision to land in early April was made on 23 March, initially the plans had been for a landing about half a year later.

This according to "The Falklands War" by Martin Middlebrook, a military historian who's studied the whole affair in depth (as much as possible, the Argentinians still often don't want to talk to British people about it all).