Conquest of the Argentine Chaco 

The Argentine Republic at the end of the 19th century and beginning of the 20th century carried out the military occupation of the Central and Southern Chaco, which until then had been in the hands of indigenous peoples. The first military expedition took place in 1870 at the end of the War of the Triple Alliance and in 1917 the conquest of the territory was concluded.
The region between the Pilcomayo, Paraguay, Paraná and Salado rivers was inhabited at the end of the 19th century by indigenous peoples:

• Guaycurúes: mocovíes, tobas and pilagáes 
• Mataco-mataguayos: wichís, chorotes and chulupíes 
• Other: vilelas, tonocotés, tapietés, chanés and chiriguanos 

El Chaco

Military expeditions and reconnaissance explorations

On April 16, 1870, Lieutenant Colonel Napoleón Uriburu left Jujuy with 250 men on mules, belonging to a regiment that he had formed with recruits from Salta and Jujuy and destined for the Orán border. He passed through La Cangayé, the old reduction of Nuestra Señora de los Dolores that had been founded in 1781 and abandoned in 1797 near the junction of the Teuco and Bermejo rivers, continued along the Bermejo and then entered the Chaco until reaching the Paraná River opposite Corrientes after 1,250 km traveled in 56 days. He subdued eleven chieftains and thousands of indigenous people who were assigned to the sugar cane harvest and reconnoitered a road to Corrientes. During this campaign, a detachment expelled a Bolivian squadron that was incurring in Argentine territory.
On February 26, 1871, the ship Sol Argentino left Buenos Aires to explore the Bermejo River as far as the province of Salta and then returned to Buenos Aires in February 1872. During this trip, there were numerous clashes with indigenous people.
President Domingo Faustino Sarmiento created the Gran Chaco National Territory with its capital in Villa Occidental (today in Paraguay) on January 31, 1872, with Julio de Vedia as its first governor.
In 1872, Uriburu traveled through the Chaco to assist the steamship Leguizamón that was stranded in the Bermejo.
In 1875, Napoleon Uriburu, already as governor of Chaco, attacked the encampments of the chiefs Noiroidife and Silketroique, defeating them. That year, the American captain Santiago Bigney and six crew members of the barge Río de las Piedras were killed by the Tobas when they were sailing along the Bermejo River and trying to trade with them. To recover the boat and another that had helped it, on December 25, 1876, Navy Captain Federico Spurr entered the Bermejo River with the Viamonte, fighting in several actions against the Tobas, whom he defeated at Cabeza del Toba. The two boats had been sunk by the natives and were recovered by Spurr with part of the cargo, arriving at Corrientes on January 17, 1877.
On July 23, 1875, Commander Luis Jorge Fontana began a reconnaissance of the entrance to the Pilcomayo River, sailing 70 km along the river.

Line Cavalry in the Southern Chaco Campaign, 1885

On April 19, 1878, Uriburu carried out a new punitive expedition.
On August 29, 1879, Colonel Manuel Obligado left Reconquista with 150 men to reconnoiter a road and returned on October 12, after traveling 750 km, without fighting the natives.
On May 4, 1880, by order of President Nicolás Avellaneda, Major Luis Jorge Fontana left Resistencia with 7 officers, 30 soldiers, 8 natives and 2 civilians with the objective of reconnoitring a road that linked Corrientes with Salta. After 104 days he arrived at Colonia Rivadavia in Salta after traveling 520 km along the Bermejo and leaving a trail open. He defeated a group of Tobas who outnumbered him in a battle in which he lost an arm. Text of the telegram sent to Avellaneda: 

I am in Rivadavia. The Chaco has been surveyed. I lost my left arm in a battle with the Indians, but I still have the other one to sign the map of the Chaco that I completed on this excursion.[1] 

On May 20, 1881, Colonel Juan Solá y Chavarría set out from the Dragones fort with 9 officers, 50 troops and 3 volunteers, with the objective of reconnoitering the interior of the area between the Pilcomayo and the Bermejo to the port of Formosa. From Fortín Belgrano he sailed along the Bermejo and, due to his delay in reaching his destination, the governor of Chaco, Colonel Bosch, sent 100 soldiers in his search. On September 3, Solá reached Herradura and from there he traveled by boat to Formosa.
On April 19, 1882, the Tobas murdered the French doctor Jules Crevaux and eleven of his companions near La Horqueta in the Pilcomayo. In mid-1882, Fontana, with the steamer Avellaneda and the launch Laura Leona, explored the Pilcomayo in search of Crevaux's remains, returning on September 18 without managing to find them. To punish the Tobas and Chiriguanos for the murder of Crevaux, Lieutenant Colonel Rudecindo Ibazeta left Dragones on June 11, 1883, leaving Dragones with 135 men. On August 10, they were attacked in the Pilcomayo by 650 partly mounted Indians, ending with the death of 60 of them. They returned on September 10 after having taught the Indians a lesson.
The French explorer Arturo Thouar made four expeditions in the Pilcomayo area in 1883, 1885, 1886 and 1892.

Victorica Campaign

In 1884, the Minister of War and Navy of President Julio Argentino Roca, General Benjamín Victorica, led a military campaign that aimed to extend the border with the indigenous people of Chaco to the Bermejo River, establishing a line of forts that would reach Salta.
Five military columns set out from Córdoba, Resistencia and Formosa with the order to converge on La Cangayé, two squadrons were to go up the Bermejo and Pilcomayo rivers and the reserve was formed by part of the Marine Infantry Regiment at Fort General Belgrano. The campaign took place between October 17 and December 21, achieving its objectives and founding three towns (in El Timbó Puerto Bermejo, Puerto Expedición and Presidencia Roca were founded, and navigation on the Bermejo River was also opened.2
On August 21, 1884, a fleet under the command of Navy Sergeant Major Valentín Feilberg left Formosa, consisting of the Pilcomayo bomber, the Explorador tug, the Atlántico steamboat, the Sara barge and another smaller one. The objective was to explore the Pilcomayo and establish a fort at its mouth. This "Coronel Fotheringham" fort is the current city of Clorinda. They explored several branches of the river up to near Salto Palmar and returned to Buenos Aires on April 14, 1885. The Swedish naturalist and hydrological engineer Olaf J. Storm participated in the expedition.
On June 25, 1885, The steamer Teuco set sail from Buenos Aires under the command of Juan Page to explore the Bermejo, returning to Corrientes on October 3.

Formosa At the End of thr 19th Century

In August 1885 a fleet of three vessels sailed along the Bermejo under the command of Guillermo Aráoz, also exploring the Teuco River. The expedition continued in January 1886 to the San Francisco River under the command of second lieutenants Sáenz Valiente and Zorrilla.
On September 19, 1886, a squadron under the command of Navy Captain Federico Wenceslao Fernández, composed of the steamer Sucre and the barge Susana, set sail from Buenos Aires with the objective of exploring the Aguaray Guazú River and verifying its links with the Pilcomayo.
On March 12, 1890, the ships Bolivia and General Paz began a new exploration of the Pilcomayo under the command of frigate captain Juan Page (who died during the exploration), exploring the Brazo Norte.
On September 1, 1899, General Lorenzo Vintter began a military campaign in the southern Chaco, commanding 1,700 men from the Chaco Operations Division, made up of an infantry battalion, five cavalry regiments, and an artillery regiment. An attempt was made to peacefully convince the indigenous people that they should submit, but several battles took place and the border line was established at the Pilcomayo River. Advanced military posts were created, communicated by telegraph and a road. The campaign concluded with the effective military occupation of the Argentine Chaco, which was carried out with little indigenous resistance.
The Chaco Cavalry Division was dissolved in 1914, leaving only the 9th Cavalry Regiment in the area.
On December 31, 1917, the Conquest of Chaco was declared over, but in March 1919 a group of Paraguayan Indians, presumably Maká,3 attacked the Yunká fort (on the Pilcomayo, in Formosa), killing the entire garrison and the inhabitants who were there, except for a soldier named Barrios who had been evacuated to Formosa, sick with malaria. He lived for many years in Clorinda, where he died in the 1970s. Today the place is called Fortín Sgto. 1º Leyes, in honor of the leader who died in that attack.

Treaties

During the Spanish colonial period, several treaties were signed with the indigenous people of Chaco:4
1662: Peace treaty between the Tocagües and Vilos Indians and Santa Fe
1710: Treaty between Governor Urizar and the Malbalaes
17??: Treaty between Governor Urizar and the Lules
1774: Treaty between Matorras and Paykin
After 1816 in the Argentine colonial period:
1822: Peace treaty between Corrientes and the Abipones
1824: Perpetual agreement between Corrientes and the Abipones
1825: Treaty between Corrientes and the Chaco Indians
1864: Agreement between the Corrientes governor Ferré and the Chaco chieftains
1872: Peace treaty between the National Government and chieftain Changallo Chico
1875: Peace treaty between the National Government and chieftain Leoncito

References 

[1] LA ARMADA ARGENTINA Y LAS CAMPAÑAS AL GRAN CHACO - 4 
[2] Expediciones y Campañas al Desierto (1820-1917) 
[3] El Río y la Frontera: Movilizaciones. Aborígenes, Obras Públicas y MERCOSUR en el Pilcomayo. Pág. 39. Autores: Gastón Gordillo, Juan Martín Leguizamón. Editor: Editorial Biblos, 2002. ISBN 9507863303, 9789507863301 
[4] Tratados en Argentina 

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