The Chaco War was fought between 1932 and 1935 between Bolivia and which other South American country?

The Chaco War (1932–1935) was fought between Bolivia and Paraguay over control of the northern part of the Gran Chaco region (known in Spanish 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 Thirst") in literary circles, for being fought in the semi-arid Chaco. It was the bloodiest military conflict fought in South America during the 20th century, between two of its poorest countries, both having previously lost territory to neighbors in 19th-century wars.

During the war, both landlocked countries faced difficulties shipping arms and supplies through neighboring countries. Bolivia faced particular external trade problems, coupled with poor internal communications. Although Bolivia had lucrative mining income and a larger, better-equipped army, a series of factors turned the tide against it, and Paraguay came to control most of the disputed zone by war's end.

The ultimate peace treaties granted two-thirds of the disputed territories to Paraguay.