Never mind direct comparisons with Portugal, the idea was that a 1982 European power would be more likely to do the calculus of utility and decide that it was not worth the treasure and blood to hit back. As pointed out the US Navy did not considet it a slam dunk by any means and Britain essentially had to throw everything it did not have tied up in NATO duty at the Falklands. *On paper *the Argentine fleet and air force should have had plentiful home waters/sky advantage, had they bothered setting up a proper operation to face a counteroffensive. However… once the actual battle is engaged the differences in training, motivation, leadership, tactical and strategic doctrine including flexibility and initiative, command/control/communications, quality vs. quantity of equipment, begin making their difference. . For all the wasteful buying of hardware to show off on parade, the military in Argentina had as its main experience crushing their own citizens.
Politically, Thatcher was unpopular at home, too, and the perception from the Argentinian side was that the fait-accompli of the taking of the South Atlantic Islands would just result in her government falling, not in a rallying-round-the-flag – because, as per the quote mentioned above by** jimbuff314**, they considered the Brits to have become corrupt, weak… one has to bear in mind, in Latin America the discourse about the decadent, corrupt, weak-spirited, materialistic, mercenary, amoral nature of the dominant US/NWEurope bloc was as much a standard propaganda line for the right as it was for the left the whole century.
usedtobe, Argentina was no classic “banana republic”. In the 60s and 70s it was the most industrialized country in South America and it was in a serious competition with Brazil as to which of them would be the Regional Power for the South Atlantic. To that end c. 1980 Argentina and Brazil both had larger-than-needed conventional militaries with operational fleet aircraft carriers (not just little Harrier jumpers, although both rather creaky), and modest fleets of guided-missile destroyers/frigates, submarines, fighter jets – and not all of it old surplus either, they had some some decently current material including Type 42 destroyers of the same class as the Sheffield; but yes, a lot of it was older mark and the way to balance things out at the time was for instance, since you can’t just buy a new carrier in time for next summer, load your fighters with new missiles, or load new missiles in box launchers on your old cruisers (the US did that with their battleships, remember?). Pinochet’s Chile was not too far behind as #3 in shows of martial capability.
But it was not completely exclusive of the would-be regional powers either - Ecuador, Peru, Venezuela themselves were bolting on Exocets or Otomats to their vessels at the time, and buying Mirages and Israeli Mirage-clones and the ocassional Soviet/Russian gear. But let’s pause now and ask how many of those antiship missiles have been fired in anger since 1982 (sadly, the Ecuadoran and Peruvian air forces *have *gotten to use their toys on one another a few times). These military pourchases were mostly intended to keep commanders happy and thinking of new hardware instead of new counter coups. The Argentines in particular did not have an adequate stock of Exocets or Super Etendards to sustain the operation. They launched the invasion *before *most of their order was delivered.