The war of Triple Alliance, also known as Paraguayan War, was the most bloody conflict of the history of Latin America. It was fought between 1864 and 1870 by Paraguay against the three allied nations: Argentine, Brazil and Uruguay.
Paraguay had been involved for years in disputes on borders and charges with its stronger neighbours: Argentine and Brazil. At the same time, Uruguayans had fought to get and maintain their Independency from the same powers, especially from Brazil.
In 1864 Brazil helped the Chief of the Uruguayan Partido Colorado, Venancio Flores, to promote a coup d’etat against the President Bernardo Prudencio Berro, member of the Partito Blanco. In response, the Paraguayan President Francisco Solano López, believing that the balance of regional power was threatened, declared war with Brazil; soon the conflict involved Argentine too, that saw its territory of the Province of Corrientes invaded by Paraguayans. Then Bartolomé Mitre, the elected President in Buenos Aires, organized an alliance with Brazil and with the Uruguay of Flores (the Triple Alliance) and on 1 May 1865 they together declared war on Paraguay.
The action of López, as a result of the constitution of an army of 50 000 men, at the time the strongest army of Latin America, was seen by many people as an aggression pursued for reasons of national and personal prestige; but with the continued war in the nations of alliance too, a large part of the public opinion felt the conflict as a war of conquest carried out by Mitre and Peter II.
At the outbreak of war, at the end of 1864, Paraguayan forces advanced northwards to the Brazilian province of Mato Grosso and southwards to the province of Rio Grande do Sul.
Logistic problems and a bigger and bigger disproportion of men and resources in favour of Allies, forced Paraguayans to retire within their borders.
In June 1865, the Brazilian Navy defeated a Paraguayan fleet in Riachuelo on the river Paraná, near the argentine city of Corrientes. By Januar 1866 Allies blocked the rivers that led to Paraguay. In April, Mitre led an allied invading army to southwest Paraguay.
Bloody battles were fought; the most important battle was won by Paraguayans in September 1866 at the fort of Curupayty and it stopped the allied offensive for almost a year. Both sides suffered heavy casualties in the campaign.
In the January 1868, Mitre was replaced as commander in chief by the Brazilian Marquis of Caxias (later Duke). In February, some Brazilian armoured vessels penetrate Paraguayan defenses, forcing the passage in front of the fluvial fortress of Humaitá, nearby the confluence between Paraná and the Paraguay river and moved forward, till they bombed Asunción.
In December, at the battle of Lomas Valentinas, the Paraguayan Army was utterly defeated.
López fled north and continued a struggle of guerrilla until his death on March 1st 1870.
The war left Paraguay in a particularly serious situation, especially from the demographic point of view: in 1871 its population of about 525 000 persons was reduced to 221 000 and men were only 28 000.
During the war Paraguayans suffered non only the enemy but also malnutrition, diseases and López’s tyranny, who killed and tortured an unlimited number of people. Argentine and Brazil annexed about 140 000 km² of Paraguayan territory: Argentine took a great part of the region of Misiones and part of the Chaco, between the rivers Bermejo y Pilcomayo; Brazil enlarged its province of Mato Grosso with the annexed territories. Both required big allowances, that were never paid and occupied Paraguay till 1876.
In the meantime “colorados” took control of Uruguay and maintained it till 1958.
The causes of the conflict remain matter of dispute among historians. Some of them attribute to Paraguay a number of social and personal successes, that ended up starting a reaction of neighbouring states and British Empire; others attribute to López and his wrong assessments in the field of foreign policy the main responsibility of the war.