The Chaco War was a border war fought from 1932 to 1935 between Bolivia and Paraguay over control of the disputed, barren Chaco Boreal region of South America. The war ended in victory for Paraguay, which acquired two-thirds of the disputed territory.
Both Bolivia and Paraguay were landlocked, Bolivia having lost its Pacific coastline to Chile during the War of the Pacific in 1879-1883. However, Paraguay enjoyed access to the Paraguay River, providing the country with access to the Atlantic Ocean. Bolivia coveted control over the river and its eventual access to the Atlantic Ocean, and, after Chilean control over Arica and the Bolivian Littoral was confirmed under the 1929 Treaty of Lima, Bolivia intensified its efforts to acquire the disputed Chaco Boreal region, which bordered the Paraguay River and the Paraguayan capital of Asuncion. Bolivia argued that the region had been part of Upper Peru, its predecessor, while Paraguay based its case on the occupation of the land. At the same time, the pro-Paraguayan Royal Dutch Shell and the pro-Bolivian Standard Oil competed for exploration and drilling rights, and Standard Oil financed Bolivia's military buildup as tensions between Bolivia and Paraguay increased.
The Bolivian Army imported the latest foreign weapons, including British light machine-guns and warplanes, while the Paraguayans used Mauser rifles that overheated easily. Paraguay's population of 880,000 was a third the size of Bolivia's, 2,150,000, but Bolivia never mobilized more than 60,000 soldiers for the war, and no more than two-thirds of its army were ever on the Chaco; Paraguay mobilized its entire army. Most Paraguayan soldiers were European-Guarani mestizos who were acclimated to the Chaco's harsh climate, while 90% of the Bolivian Army consisted of Quechua or Aymara peasants from the Altiplano who had never been to the hot, humid Chaco.
On 15 June 1932, a Bolivian detachment burned Fortín Carlos Antonio López at Pitiantutá Lake, despite President Daniel Salamanca giving strict orders that the detachment was not to provoke Paraguay. On 16 July, a Paraguayan detachment drove the Bolivian troops from the area. Salamanca proceeded to order the capture of three Paraguayan outposts, and Paraguay responded by calling for a Bolivian withdrawal. Bolivia instead reinforced its 1st Army to 4,000 soldiers, while Paraguay mobilized 10,000 troops in August and sent them into the Chaco.
The first Paraguayan offensive culminated in the Battle of Boqueron, at which the Paraguayans besieged a Bolivian-held fort for 22 days. afterwards, the Paraguayans launched a pincer maneuver to attack the remaining Bolivians, only to find that they had withdrawn. By December 1932, Bolivia was fully mobilized for war. General Hans Kundt, a German World War I veteran, was entrusted with commanding the Bolivian counteroffensive. In January 1933, he attacked Fortín Nanawa, hoping to endanger the Paraguayan city of Concepcion and reach the Paraguay River. Though the Bolivians lsot the First Battle of Nanawa, they took Fortín Alihuata and cut the supply route of the Paraguayan 1st Division, which they proceeded to rout at the Battle of Campo Jordan. In July 1933, the Paraguayans again defeated a Bolivian attack on Nanawa, and the Bolivians lost 2,000 men to Paraguay's 559. The Paraguayans regained the strategic initiative and began a new offensive in September. Three Bolivian regiments were encircled in the Campo Via pocket and forced to surrender, and Jose Felix Estigarribia was promoted to general for his victory. The Paraguayans proceeded to recapture Fortín Alihuata in December 1933, and Kundt was forced to resign as chief of staff of the Bolivian Army. On 19 December 1933, a 20-day ceasefire was called.
By January 1934, Bolivia had reorganized its eroded army and assembled a larger force. The Paraguayans launched their third offensive on the armistice's end, forcing the Bolivians to abandon their defensive line at Magariños-La China. In May 1934, the Bolivians ambushed the advancing Paraguayans at the Battle of Cañada Strongest and inflicted a major defeat, capturing 67 officers and 1,389 soldiers. The Paraguayans continued to encircle and destroy Bolivian divisions as the year went on, however, forcing the Bolivians to retreat. The Bolivian generals deposed Salamanca as he visited their camp and replaced him with Vice President Jose Luis Tejada Sorzano. The Bolivian cavalry corps scored its first victory on 9 November 1934, putting the Paraguayan Army on the run, but they soon found themselves exhausted and thirsty after advancing into terrain where the Paraguayans controlled all water access. In December 1934-Janaury 1935, the Paraguayans launched a new offensive that killed 200 Bolivians and captured 1,200 losses with just a few dozen Paraguayan casualties. The Bolivians prepared for a last stand at Villa Montes, and the Paraguayan general Jose Felix Estigarribia launched a final assault on the Bolivians on 7 February 1935. The Paraguayans were forced to retreat after a series of failed attacks, and Bolivian counteroffensives forced the Paraguayans back. Another ceasefire was signed on 12 June 1935. By then, Paraguayan troops had been making preparations to attack Bolivia's oil fields in Cordillera Province.
The conflict ended with a comprehensive Paraguayan victory, and two-thirds of the disputed region would come under Paraguayan control under a 1938 truce signed in Buenos Aires. Most of the war's 100,000 casualties were caused by malaria and infections rather than from combat-related causes. Bolivia's blunders during the war led to the rise of Rafael Franco's socialist regime and the left-wing nationalist Revolutionary Nationalist Movement, while the war recharged Paraguayan nationalism and led to the resurgence of the Colorado Party of Paraguay.