Here's what I think what happened to where I am, especifically, Belém, Pará:
Brazil: Brazil was nuked, but their enemies didn't have the ammount of nukes required to totally anihilate it. All big capitals and big cities were attacked, but the interior was mostly intact, suffering the ravages of radiation, chaos and mutation instead.
What remained of the brazilian army weathered the storm, but the central governanment was apparently wiped out. That means that, although the nation of Brazil still exists, central authority is limited to some areas controlled by the armies of Brazil. In the local context, authority was passed to the "Colonels", leaders of local militias of various origins, who report to the military. The idea is that post-nuclear Brazil is like a Coronelist and secular version of the Holy Roman Empire. The military is divided in multiple factions. Some believe in restoring central authority here and now, others believe in keeping descentralization, others believe what remains of the army should invade other countries to use the destruction of Brazil's enemies and rivals to their advantage, etc.
The city of Belém (a large city of three millions at the time) and the metropolitan region (Ananideua, Marituba, Benevides, Barcarena, Santa Isabel) was the target of nuclear strikes from the enemies of Brazil. The destruction of land transports in amazon meant that the local peoples of the Amazon returned to using rivers as their main form of transport and communication. The roads and highways were abandoned to the wilderness.
These city-states are usually surviving countryside cities near rivers, whose resources, industry and localization allow them to keep a semblance of working society. The city-states are independent from one another, and seek to co-opt the smaller villages and towns into their domination or "protection", as a way of acquiring more resorces, troops, technology and power to use against the other city-states.
Beyond the rivers and the coast-line, live only military enclaves, isolated towns and amerindian tribes. Mutated by radiation, the jungle turned into a ever-growing vegetation that constantly threatens to overwhelm the cities. The animals of the jungle included mutated army ants (imagine New Vegas ants, now make them orange and make them into a immense swarm that devours everything), Jaguars (biting and scratching machines of death, about the size of deathclaws) and Cobra Grandes (IMMENSE cobras that eat people and spit poison) and the ever-present Radscopion, except that the one in amazon is the most poisonous in all of Brazil, so it makes the FO radscorpion look harmless.
The city of Belém and its metropolitan region is now a lawless land. After the radiation went away, scavengers moved in. Then the City-States decided to move after Belém in a effort to find ancient technology and salvage it to obtain greater firepower, better medicine and more. Currently, Belém is a battleground between factions of scavengers, raiders and other criminals, along with some troops from the city-states helping their local allies. All city-states seek to reclaim Belém and declare themselves rightful rulers of the land, but to do that requires great power, power that can be only found in old technology.
The archeotypal "hero" (think Mad Max) of this land is a small (1,55m-1,65m) brown man of luso-indian-african features and dark brown eyes, wearing a light sleeveless shirt with a old police bulletproof vest over it, jeans shorts and military boots. His head is covered by a strawn hat. His weapons are a spear, two taurus .38 handguns in his holsters and a shotgun at his back.