The origin of the war was a border dispute between Bolivia and Paraguay over the Chaco. This vast area was largely undeveloped except for some minor oil discoveries by Standard Oil in Bolivia and Royal Dutch Shell in Paraguay. The Chaco, which Bolivia traditionally regarded as a province (Gran Chaco), became more significant to Bolivia after the latter lost its Pacific Ocean outlet to Chile. Bolivia hoped to gain access to the Atlantic Ocean with an oil pipeline across the Chaco to the Paraguay River. Despite mediation attempts by various countries, the increased number of border incidents led the military high commands of Bolivia and Paraguay to believe in the inevitability of war.

Salamanca used one of the border incidents to break diplomatic relations with Paraguay and increase Bolivia's military budget, even though the country had severe economic problems. Convinced that Bolivia's better-equipped, German-trained troops, which outnumbered the Paraguayan army, could win the war, Salamanca went to war in 1932.

The war raged for the next three years. The Bolivians were defeated in all major battles, and by the end of 1934 they had been driven back 482 kilometers from their original positions deep in the Chaco to the foothills of the Andes. Serious strategic errors, poor intelligence, and logistical problems in reaching the distant battle lines contributed to the losses. In addition, the morale of the Bolivian troops was low, and the highland Indians could not adapt to the extreme climate in the low-lying Chaco. Despite the high command's decision to end the war, Salamanca was determined to continue at all costs. In 1934, when he traveled to the Chaco to take command of the war, Salamanca was arrested by the high command and forced to resign. His vice-president, José Luis Tejada Sorzano, who was known to favor peace, was accepted as president (1934-36).

Salamanca's overthrow was a turning point in the Chaco War. The Paraguayan troops were stopped by new, more capable Bolivian officers, who fought closer to Bolivian supply lines. On June 14, 1935, a commission of neutral nations (Argentina, Brazil, Chile, Colombia, Peru, and the United States) declared an armistice; a definite settlement was finally reached in 1938. Bolivia lost the Chaco but retained the petroleum fields, which Paraguay had failed to reach. Both countries suffered heavy losses in the war. In Bolivia alone, an estimated 65,000 people were killed and 35,000 wounded or captured out of a population of just under 3 million.

The humiliating disaster of the Chaco War had a profound impact in Bolivia, where it was seen as dividing the history of the twentieth century "like a knife." The traditional oligarchy was discredited because of its inept civilian and military leadership in the war. Unable to deal with growing criticism, its members blamed the loss of the war on the low potential of the Bolivians and saw the earlier pessimistic assessment in Arguedas's famous novel Pueblo Enfermo (A Sick People) confirmed.

After the war, a group of middle-class professionals, writers, and young officers questioned the traditional leadership. This group, which came to be known as the "Chaco Generation," searched for new ways to deal with the nation's problems. It resented the service of the rosca on behalf of the tin-mining entrepreneurs and criticized Standard Oil, which had delivered oil to Paraguay clandestinely through Argentine intermediaries during the war. The Chaco Generation was convinced of the need for social change. Gustavo Navarro, now more radical than during the 1920s, raised the famous slogan "land to the Indians, mines to the state." The military, which came to power in 1936, tried to bring about change with popular support.

Data as of December 1989