The Chaco War (Spanish: Guerra del Chaco, Guarani: Cháko Ñorairõ) was fought from 1932 to 1935 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. The war is also referred to as La Guerra de la Sed (Spanish for "The War of Thirst") in literary circles since it was fought in the semi-arid Chaco. The bloodiest interstate military conflict fought in South America in the 20th century, it was fought between two of its poorest countries, both of which had lost territory to neighbors in 19th-century wars.

During the war, both landlocked countries faced difficulties shipping arms and supplies through neighboring countries. Bolivia, in particular, faced external trade problems and with poor internal communications. Although Bolivia had lucrative mining income and a larger and better-equipped army, a series of factors turned the tide against it, and Paraguay controlled most of the disputed zone when the war had ended.

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