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The Paraguayan War,[A] also known as the War of the Triple Alliance[B] and the Great War in Paraguay,[C] was a South American war fought from 1864 to 1870 between Paraguay and the Triple Alliance of Argentina, the Empire of Brazil, and Uruguay. It was the deadliest and bloodiest inter-state war in Latin America's history. It particularly devastated Paraguay, which suffered catastrophic losses in population: almost 70% of its adult male population died, according to some counts, and it was forced to cede territory to Argentina and Brazil. According to some estimates, Paraguay's pre-war population of 525,000 was reduced to 221,000, of which only 28,000 were men.
The war began in late 1864, as a result of a conflict between Paraguay and Brazil caused by the Uruguayan War. Argentina and Uruguay entered the war against Paraguay in 1865, and it then became known as the "War of the Triple Alliance".
The war ended with the total defeat of Paraguay. After it lost in conventional warfare, Paraguay conducted a drawn-out guerrilla resistance, a disastrous strategy that resulted in the further destruction of the Paraguayan military and much of the civilian population through battle casualties, hunger and diseases. The guerrilla war lasted 14 months until President Francisco Solano López was killed in action by Brazilian forces in the Battle of Cerro Corá on 1 March 1870. Argentine and Brazilian troops occupied Paraguay until 1876. Estimates of total Paraguayan losses range from 21,000 to 200,000 people. It took decades for Paraguay to recover from the chaos and demographic losses.
The Platine region in 1864. The shaded areas are disputed territories.
Since their independence from Portugal and Spain in the early 19th century, the Empire of Brazil and the Spanish-American countries of South America were troubled by territorial disputes. All nations in the region had lingering boundary conflicts with multiple neighbors. Most had overlapping claims to the same territories. These issues were questions inherited from their former metropoles, which, despite several attempts, were never able to resolve them satisfactorily. Signed by Portugal and Spain in 1494, the Treaty of Tordesillas proved ineffective in the following centuries as both colonial powers expanded their frontiers in South America and elsewhere. The outdated boundary lines did not represent actual occupation of lands by the Portuguese and Spanish.
By the early 1700s, the Treaty of Tordesillas was deemed all but redundant and it was clear to both parties that a newer one had to be drawn based on realistic and feasible boundaries. In 1750, the Treaty of Madrid separated the Portuguese and Spanish areas of South America in lines that mostly corresponded to present-day boundaries. Neither Portugal nor Spain were satisfied with the results, and new treaties were signed in the following decades that either established new territorial lines or repealed them. The final accord signed by both powers, the Treaty of Badajoz (1801), reaffirmed the validity of the previous Treaty of San Ildefonso (1777), which had derived from the older Treaty of Madrid.
The territorial disputes became worse when the Viceroyalty of the Río de la Plata collapsed in the early 1810s, leading to the rise of Argentina, Paraguay, Bolivia and Uruguay. Historian Pelham Horton Box writes: "Imperial Spain bequeathed to the emancipated Spanish-American nations not only her own frontier disputes with Portuguese Brazil, but problems which had not disturbed her, relating to the exact boundaries of her own viceroyalties, captaincies general, audiencias and provinces." Once separated, Argentina, Paraguay and Bolivia quarreled over lands that were mostly uncharted and unknown. They were either scarcely populated or settled by indigenous tribes that answered to no parties. In the case of Paraguay with her neighbor Brazil, the problem was to define whether the Apa or Branco rivers represented their actual boundary, a persistent issue that also confused Spain and Portugal in the late 18th century. The region between both rivers was depopulated, except for some tribes that roamed the area attacking nearer Brazilian and Paraguayan settlements.
There are several theories regarding the origins of the war. The traditional view emphasizes the policies of Paraguayan president Francisco Solano López, who used the Uruguayan War as a pretext to gain control of the Platine basin. This caused a response from the regional hegemons Brazil and Argentina, who exercised influence over the much smaller republics of Uruguay and Paraguay.
The war has also been attributed to the after-effects of colonialism in South America, with border conflicts between the new states, the struggle for power among neighboring nations over the strategic Río de la Plata region, Brazilian and Argentine meddling in internal Uruguayan politics (which had already caused the Platine War), and Solano López's efforts to help his allies in Uruguay (previously defeated by the Brazilians), as well as his presumed expansionist ambitions.
Before the war Paraguay had experienced rapid economic and military growth as a result of its protectionist policies which had boosted the local industry (much to the detriment of British imports). A strong military was developed because Paraguay's larger neighbors Argentina and Brazil had territorial claims against it and wanted to dominate it politically much like they did in Uruguay. Paraguay had recurring boundary disputes and tariff issues with Argentina and Brazil for many years during the rule of Carlos Antonio López.
Since Brazil and Argentina had become independent, their struggle for hegemony in the Río de la Plata profoundly marked the diplomatic and political relations among the countries of the region.
Brazil was the first country to recognize the independence of Paraguay in 1844. At this time Argentina still considered it a break-away province. While Argentina was ruled by Juan Manuel Rosas (1829–1852), a common enemy of both Brazil and Paraguay, Brazil contributed to the improvement of the fortifications and development of the Paraguayan army, sending officials and technical help to Asunción.
Brazil had carried out three political and military interventions in the politically unstable Uruguay: in 1851 against Manuel Oribe in order to fight Argentine influence in the country and to end the Great Siege of Montevideo; in 1855, at the request of the Uruguayan government and Venancio Flores, leader of the Colorado Party, who were traditionally supported by the Brazilian empire; and in 1864, against Atanasio Aguirre. This last intervention would lead to the Paraguayan War.
On April 19, 1863, Uruguayan General Venancio Flores, who was then an officer in the Argentine army and the leader of the Colorado Party of Uruguay, invaded his country, starting the Cruzada Libertadora, with open support of Argentina which supplied rebels with arms, ammunition and 2,000 men. Flores wanted to overthrow the Blanco Party government of President Bernardo Berro,:24 which was allied with Paraguay.:24
Paraguayan President López sent a note on 6 September 1863 to the Argentine government asking for an explanation, but Buenos Aires denied any involvement in Uruguay.:24 From that moment, mandatory military service was introduced in Paraguay and in February 1864, an additional 64,000 men were drafted into the army.:24
One year after the beginning of the "Cruzada Libertadora", in April, 1864, Brazilian minister José Antônio Saraiva arrived in Uruguayan waters with the Imperial Fleet, to demand payment for damages caused to gaucho farmers in border conflicts with Uruguayan farmers. Uruguayan President Atanasio Aguirre from the Blanco Party rejected the Brazilian demands, presented his own demands and asked Paraguay for help. To settle the growing crisis, Solano López offered himself as mediator of the Uruguayan crisis, as he was a political and diplomatic ally of the Uruguayan Blancos, but the offer was turned down by Brazil.
Brazilian soldiers on the northern borders of Uruguay started to provide help to Flores' troops, harassed Uruguayan officers, while the Imperial Fleet pressed hard on Montevideo. During the months of June–August, 1864 a Cooperation Treaty was signed between Brazil and Argentina at Buenos Aires, for mutual assistance in the Plate Basin Crisis. 
Brazilian Minister Saraiva on August 4, 1864, sent an ultimatum to the Uruguayan government: either comply with the Brazilian demands, or the Brazilian army would retaliate. The Paraguayan government was informed of all this and sent to Brazil a message, which stated in part:
The government of the Republic of Paraguay will consider any occupation of the Oriental territory [i.e. Uruguay] as an attempt against the equilibrium of the states of the Plate which interests the Republic of Paraguay as a guarantee for its security, peace, and prosperity; and that it protests in the most solemn manner against the act, freeing itself for the future of every responsibility that may arise from the present declaration.
— José Berges, Paraguayan chancellor, to Vianna de Lima, Brazilian minister to the Paraguayan government. August 30, 1864.
The Brazilian government, probably believing that the Paraguayan threat would be only diplomatic, answered in September 1, 1864, stating that "they will never abandon the duty of protecting the lives and interests of Brazilian subjects". But in its answer two days later Paraguayan government insisted that "if Brazil takes the measures protested against in the note of August 30th, 1864, Paraguay will be under the painful necessity of making its protest effective."
Despite the Paraguayan notes and ultimatums, on October 12, 1864 Brazilian troops under the command of Gen. João Propício Mena Barreto invaded Uruguay:24 thus marking the beginning of the hostilities.  Paraguayan military actions against Brazil began on 12 November 1864, when the Paraguayan ship Tacuarí captured the Brazilian ship Marquês de Olinda, which had sailed up the Paraguay River to the province of Mato Grosso, with the Province's newly appointed President on board. Paraguay would officially declare war on Brazil only on December 13, 1864, on the eve of the Paraguayan invasion on the Brazilian province of Mato Grosso.
The conflict between Brazil and Uruguay was settled in February 1865. News of the war's end was brought by Pereira Pinto and met with joy in Rio de Janeiro. Brazilian Emperor DomPedro II found himself waylaid by a crowd of thousands in the streets amid acclamations. But public opinion quickly changed for the worse when newspapers began running stories painting the convention of 20 February as harmful to Brazilian interests, for which the cabinet was blamed. The newly raised Viscount of Tamandaré and Mena Barreto (now Baron of São Gabriel) had supported the peace accord. Tamandaré changed his mind soon afterward and played along with the allegations. A member of the opposition party, José Paranhos, Viscount of Rio Branco, was used as a scapegoat by the Emperor and the government and was recalled in disgrace to the imperial capital. The accusation that the convention had failed to meet Brazilian interests proved to be unfounded. Not only had Paranhos managed to settle all Brazilian claims, but by preventing the death of thousands, he gained a willing and grateful Uruguayan ally instead of a dubious and resentful one, which provided Brazil with an important base of operations during the acute clash with Paraguay that shortly ensued.