The Chaco War (Spanish: Guerra del Chaco, Guarani: Cháko Ñorairõ) was fought from 1932 to 1935. It was between Bolivia and Paraguay over control of the northern part of the Gran Chaco region (known in Spanish as the Chaco Boreal), which was thought to be rich in petroleum. The war is also referred to as La Guerra de la Sed (Spanish for "The War of Thirst"), since it was fought in the semi-arid Chaco. It was the first South America war in which modern weapons (such as machine guns, armoured fighting vehicles and airplanes) were used, and also the bloodiest South America war of the 20th century — around 2% of the Bolivian population and 3% of Paraguayans were killed during the conflict.
During the war, both landlocked countries faced difficulties moving arms and supplies through neighbouring countries. Despite its income from mining and a larger and better-equipped army, problems with international trade and poor internal communications ultimately turned the tide against Bolivia. The war concluded at the Chaco Peace Conference in Buenos Aires in July 1938, at which both countries signed a peace treaty awarding three-quarters of the Gran Chaco to Paraguay.
