I know the US had Plan Rubber, which was a proposed operation to invade Brazil in 1942. They were part afraid of Brazil supporting the Axis, the other part was them wanting to use Brazilian bases.
Well, I only knew about an American war plan (against Brazil) which perhaps, I'm confusing it with someone else, but I think that was the
'Pot of Gold' plan...Perhaps the plan was renamed or was a different one?
This plan envisaged transporting 10,000 US soldiers in 150 planes to Brazil, to prevent to the Nazis from occupying Brazilian territory.
The plan was the result of widespread fear, at these times, about the possibility that a Nazi invasion and/or coup would happen... both in Brazil and/or its neighboring countries.
It originated, too, in the Germany's military and logistical capabilities' overestimation.
Within that context and as stated in a
book written years ago, by a former Uruguayan minister (who would have had access to the original sources in the US archives), there would have been one possibility that would have started a war in South America.
This war would be caused by
an official request of the United States for get authorization to build and operate a military base (guess that would be an Naval Air Station) in the Río de la Plata.
Beside of its, at least, problematic strategic utility, at least for the purposes publicly announced, by the USA, of fighting against Nazi Germany.
Clearly, in my opinion, it's only use would have been its use for prevent or counteract an Argentina explicitly aligned and / or allied with the Axis.
That possibility that arose first as a hypothesis of conflict and after that on what the USA should do to be able to deploy and/or operate their forces in this, potential, new theater of war,
in the Río de la Plata.
This war hypothesis seems to have been created by two officers of the United States Navy: Albert Benjamin and William Brereton, who would have signed a confidential memorandum on the subject.
The place proposed / selected in that memorandum, would have been the Laguna del Sauce, near to ,the then, depopulated Punta del Este, (on the border between the Rio de la Plata and the Atlantic Ocean) could be an ideal place to base and, from there deploy, hydroplanes, ready to operate in the Rio de la Plata.
Of course it goes without saying that, although for different reasons, both
the political opposition in Uruguay and the Argentine government of the time were alarmed and provoked their immediate rejection of the prospect that the US Navy would set up a naval air base in the region.
According to the author, the English ambassador Eugen Millington Drake, correspondents of the New York Times and mainly Edwin Carleton Wilson, head of the US mission in Uruguay and manager of the negotiation for built the bases, had intentionally built the fear of the "Nazi plot" with the aim of defending that area of America through military bases to be installed in Uruguay.
According to A. Mercader, "the Nazi scarecrow was rising, while plans were being made to propose a deal that would include the construction of military bases on Uruguayan territory.
Bases that would have been controlled totally or partly by the United States, in this way Nazis and bases were linked from the beginning. that had its logic: ''to 'defend' Uruguay from the Nazis 'nothing better' than to build the bases with the help of a powerful ally "
It is even mentioned (in the book) as an extra source an operations manual of the FBI, published in 1942 ,especially for agents destined to Uruguay. 'This FBI manual' would reproduce the aforementioned hypothesis of conflict for the Río de la Plata.
Which, although without defining the identity from those who could control the region militarily, (supposedly) it would support that: "A small but well-organized military force in Punta del Este or Montevideo could control the mouth and the access channel to the Río de la Plata, effectively blocking all Argentine trade through the ports of Buenos Aires and Rosario... especially if an effective aviation is available."
Given that I think that if the authorization to build it by the Uruguayan parliament of the time and before a threat of this magnitude for Argentina, wouldn't be rejected ... It is very possible that (ironically), only the authorization to build it, could have created the conflict to prevent which, supposedly, would have been projected the US base.
Useful info about the WWII South America Military.