More than 80 years ago, Paraguay and Bolivia fought one of the most important wars to take place in 20th century South America. Between 1932 and 1935, the outcome of the Chaco War  determined who controlled the region known as Northern Chaco , which sets the border between Paraguay and Bolivia. Both countries mobilized roughly 400,000 soldiers for this three-year conflict.
The war in Chaco came almost 70 years after the War of the Triple Alliance  (also known as the Paraguayan War), where Brazil, Uruguay, and Argentina devastated Paraguay. The conflict lasted over six years, beginning in 1864 and ending in 1870. When it was over, Paraguay was stripped of much of the territory it disputed with Brazil and saddled with huge war reparations.
En 1932, el gobierno boliviano de Daniel Salamanca decidió ir a la guerra contra el Paraguay. […] Creía que una pronta victoria le permitiría afrontar distintos frentes: el interno y el externo.
En el primero de ellos, aunó esfuerzos en una unidad patriótica y aprovechó para atacar opositores y cerrar sindicatos que denunciaban que el costo de la crisis del 29 lo estaba pagando el pueblo en su conjunto. En el frente externo, se proponía hacerse con el Chaco Paraguayo, y con ello de una salida fluvial, luego de haber perdido tres décadas atrás la salida al océano Pacífico con Chile .
In 1932, the Bolivian administration of Daniel Salamanca decided to wage a war against Paraguay. […] They believed a quick victory will allow them different fronts: internal and external.
On the former, they brought efforts together for a patriotic unity and used the opportunity to attack opponents and close down trade unions that denounced that the people altogether were paying the cost of the Great Depression. On the latter, they were determined to obtain the Paraguayan Chaco, thus gaining a river way, after losing three decades earlier their access to the Pacific Ocean to Chile .
About Paraguay, the website says:
Los paraguayos no buscaron la guerra, pero volverían a mostrar que, pese a los históricos embates de sus vecinos […] tendrían con qué responder. […] el avance boliviano le llevó a decretar la movilización general.
La contraofensiva paraguaya, cuyo punto de inflexión se vivió en la Batalla del Boquerón (septiembre de 1932), le permitió recuperar los fuertes fronterizos y, pronto, ingresar a la región andina, donde se detuvo para el inicio de las arduas negociaciones que siguieron.
Paraguayans weren't looking for war, but they were willing to show that, in spite of the historic attacks from their neighbors […] they would have something with which to answer […] the Bolivian progress made them order a general mobilization.
The Paraguayan counter-offensive, launched with the Battle of Boquerón (September 1932), allowed them to recover the border forts, and soon after that, to get into the Andean region, where it stopped to start the arduous negotiations that came after that.
On June 14, 2015, Bolivian President Evo Morales and Paraguayan President Horacio Cartes met to commemorate  the 80th anniversary of the end of the Chaco War:
En el aniversario, ambos gobiernos destacan que el futuro de ambos países yace en la integración, impulsada por vientos de paz en la región sudamericana.
Este domingo, en el corazón del Chaco, Villa Montes, los gobiernos de Morales y Cartes darán impulso al proceso de integración física vecinal con la premisa de generar beneficios para dos pueblos sin mar y que luchan por salir de la pobreza.
On the anniversary, both governments highlight that the future for our countries lies in integration, powered by winds of peace in the South American region. […] On Sunday [June 14], in the heart of el Chaco, Villa Montes, Morales’ and Cartes’ administrations will drive the physical integration process with the neighbors with the goal of generating benefits for both landlocked territories who fight for defeating poverty.
As noted by the website Infonews, the war is also remembered for the economic interests  of foreign governments and private businesses that played a role in the conflict, which Infonews calls a “bloody and useless war”:
En el conflicto también intervinieron potencias internacionales. Principalmente, Gran Bretaña y Estados Unidos, que vendieron armamento y municiones a Bolivia y Paraguay por igual. […] la rivalidad que por aquella época existía entre la petrolera estadounidense Standard Oil, que operaba en suelo andino, y la británica Royal Dutch Shell, que promovía sus intereses desde el lado paraguayo.
Lo paradójico es que, una vez finalizado el conflicto, ninguno de los dos países logró sus objetivos iniciales. Ni Paraguay consiguió capturar la zona petrolera boliviana en el río Parapetí y sus adyacencias, ni Bolivia pudo expandir su territorio hasta las márgenes del río Paraguay, donde apenas obtuvo un puerto franco y libre tránsito para sus mercaderías, todo acordado en el tratado final de paz de 1938.
Some international powers also took part in the conflict—mainly Great Britain and United States, which sold weapons and ammunition to Bolivia and Paraguay alike. […] the rivalry that existed back then between the American oil company Standard Oil, which operated in Andean territory, and the British Royal Dutch Shell, which promoted its interests from the Paraguayan side.
What's paradoxical is that, once the conflict ended, neither of the two countries achieved their initial goals. Paraguay could not capture the Bolivian oil region on the Parapetí river and its surroundings, and neither was Bolivia able to expand its territory to the banks of the Paraguay river, where won only a free port and free transit for their goods—all agreed upon on the final peace treaty signed in 1938.
On Twitter, the users remembered the sacrifice by the Bolivian family Campero, which sent out its five sons to the conflict:
La familia que mandó a sus cinco hijos a la guerra. Hermanos Campero fueron a la guerra por amor a la patria. http://t.co/pNwkfPWNrT 
— Daniel Antonio (@dvelasquez38) junio 15, 2015 
The family that sent out its five sons fight in the war. The Campero brothers went to war out of love to the country.
Fernando Campero Paz, the son of one of the Campero brothers , also tweeted about his family's service to the nation:
Such an honor. The five Campero Trigo brothers defended the Motherland. Article on BBC World!!!
According to BBC World , a recent survey in March 2015 by the Confederation of Veterans of the Chaco War (known as Conexchaco) found out that roughly 105 veterans of the Chaco War's 180,000 soldiers are still alive in Bolivia today. These men are close to 100 years old.
In 1938, through a secret treaty  signed in July 9, 1938, Paraguay gave up 110,000 square km occupied by its army at the end of the hostilities. The Treaty of Peace, Friendship, and Borders was signed in July 21, 1938. Many years later, on April 27, 2009, the definite borders treaty was settled. The old disputed region was divided, and three fourths of it remained under Paraguayan sovereignty, with the other fourth part remaining under Bolivian control. Bolivia received territory on the banks of the upper Paraguay river.