South America
From Halopedia, the Halo wiki
South America is a continent of the planet Earth. It is situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere. It is bordered on the west by the Pacific Ocean and on the north and east by the Atlantic Ocean; North America and the Caribbean Sea lie to the northwest. South America has an area of 17,840,000 square kilometres (6,890,000 sq mi), or almost 3.5% of the Earth's surface.
The Rainforest Wars
South America was the site of the Rain Forest Wars of the Interplanetary War during 2162. The war was fought between the UN (backed by the UN Council), and the Koslovics and Frieden rebel factions. The war brought a great deal of suffering to the people of Earth, including population surges and famines.
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Sources
http://www.xbox.com/en-US/games/h/halo/storyline.htm - Xbox.com Backstory