The Chaco War was fought from 1932 to 1935. It was between Bolivia and Paraguay over control of the northern part of the Gran Chaco region, which was thought to be rich in petroleum. The war is also referred to as La Guerra de la Sed, since it was fought in the semi-arid Chaco. It was the first that took place in South America in which modern weapons 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.