The Gran Chaco or Chaco Boreal is a large lowland plain that today is part of Brazil, Bolivia, Paraguay and Argentina. 

From the late nineteenth century, there had been simmering tensions between Paraguay and Bolivia over their control of the area, as the land was of key strategic interest to both countries. It’s not unrelated that Paraguay and Bolivia are the only two landlocked countries in South America. 

The situation wasn’t helped when speculation arose that the region could hold significant deposits of oil. This led to major oil companies taking sides, with Royal Dutch Shell backing Paraguay and Standard Oil supporting Bolivia.