If Uruguay and Paraguay join this country, Brazil may declare war just because of how good this country's football team would turn out to be.
But seriously, Brazil has historically had a lot of influence over Uruguay, so there realistically could be a lot of war between these two countries. If the Sudamericanos become expansionist, they could hold real naval supremacy on two oceans as far as South America goes, including a huge fleet based out of Rio de la Plata. An advance up the coast into southern Brazil wouldn't be difficult with a large enough fleet there to support it.
This is suddenly a very interesting timeline. The geography of this nation makes it one that could easily become the regional hegemon, especially in the days preceding the Panama Canal.