Patagonia was regarded as terra nullius by Britain and France until 1881 (and thus shown as a separate state in most maps of the period). Their main concern, shared with the United States, was to prevent any nation restricting navigation through the Straits of Magellan.
Facundo Machaín and Bernardo de Irigoyen signed a treaty in Buenos Aires on behalf of Paraguay and Argentina, concluding peace between the two countries in the aftermath of the Paraguayan War. By the terms of the treaty, Paraguay ceded its claims to Misiones Province and all the lands south of the Pilcomayo River to Argentina. However, under pressure from Brazil, Argentina abandoned its claim to Villa Occidental and left its dispute with Paraguay over the northern Chaco to US arbitration.
in wikipedia
22 Jun 1876–14 May 1879 Allied withdrawal from Paraguay
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After its 1872 treaty with Paraguay, the Empire of Brazil began withdrawing its forces from that country, with the last Brazilian troops departing in June 1876. The Argentine occupation of southern Paraguay would last for several more years and it was only in 1879—after US President Rutherford Hayes awarded the northern Chaco to Paraguay—that Argentina would remove the last of its forces.
in wikipedia
