WesWorld

Argentina and Brazil are close allies within the ABC Treaty. There exists naval co-operation, in effect Argentina would look after the south and Brazil the north. Also, Chile would guard the Horn. Generally in the latter stages of the 1935 War that is how things turned out, but Brazil's Navy by then had been hammered in its own bases and had become little more than a coastal defence force and Chile's neutrality in the early stages meant that Argentina had to escort around the Horn to some extent into the South Pacific but not too far given the short range and inadequate numbers of escorts.

Remember while the Argentine, Brazilian and SAE navies all lie at anchor within easy reach of each others powerful land-based air forces, the RSAN has ample ships at home to send across the Atlantic. Brazil was knocked out at anchor within days and took damage that took years to repair within the fleet. While similar attacks could cripple the RSAN in Grand Uruguay, within a week reinforcements would arrive equal in strength to what is already in South America.