The Chaco War (1932–1935) () was fought between Bolivia and Paraguay over control of the northern part of the Gran Chaco region (known as Chaco Boreal) of South America, which was thought to be rich in oil. It is also referred to as La Guerra de la Sed (Spanish for "The War of the Thirst") in literary circles for being fought in the semi-arid Chaco. The war was the bloodiest military conflict fought in South America during the 20th century. The war pitted two of South America's poorest countries, both having previously lost territories to neighbors in wars during the 19th century. During the war both countries faced difficulties in obtaining arms and other supplies since their landlocked situation made their foreign trade and arms purchases dependent on the willingness of neighboring count