Political map of South American nations 15 April 1935 (Paraguayan Chaco Offensive): The first year of the Chaco War favored the Bolivians, but when their offensive collapsed in late 1933, Paraguay seized the initiative. By 1935, the Paraguayans had conquered most of the Chaco region. Nonetheless, both nations suffered heavily, each losing 2-3% of their population (some 100,000 over all). A treaty in 1938 confirmed Paraguay's gains, but it would not be until 2012 when commercial amounts of oil would be discovered in the territory.