Brazil (or at least, a "Brazil"-like nation state)...although this has to push "WW I" to the Eighteenth Century.
Departure point is very early - Treaty of Tordesillas line is east-west, rather than north-south, perhaps - and the end result is that by the end of the Anglo-French wars (i.e. World War I) in the early Nineteenth Century, there are essentially three powers in the Western Hemisphere. North to south:
U.S - anglophone federal republic; includes what would have become BNA (former New France, Acadia, etc) plus what we would see as the US of 1820 or so;
Mexican Empire - includes Central America and the Caribbean; largely Spanish-speaking;
Brazilian Empire - includes the entirety of South America; largely Portuguese speaking.
Potential zones of conflict are the northern frontiers of Mexico/western frontiers of the US; southern Central America/northern South America and West Indies; and the various remnants of the European colonial empires, plus frontiers with the more developed indigenous societies.
Europeans are, presumably, exhausted/focused on Europe and the Med after (essentially) five decades of warfare.
Could make for an interesting Nineteenth Century for all concerned.

Best,