I can't find it right now, so I'll say my source is "it came to me in a dream", but IIRC some US govt bodies had a scenario where if the amount of nukes is not enough to wipe out all life on Earth, but NATO and the WP takes each other out, they mostly make the Northern Hemisphere plus Australia and likely India uninhabitable, so the biggest remaining military power would be Brazil, with the remaining habitable places being Africa and South America.