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Paraguayan War (1864–1870)

On 12 October 1864, despite Paraguayan ultimatums, Brazil (allied with the Argentine Government under General Bartolomé Mitre and the rebellious Uruguayan colorados led by Gen. Venancio Flores) invaded the Republic of Uruguay in order to overthrow the government of that time (which was under the rule of the Blanco Party, an ally of López), thus starting the Paraguayan War.

The Paraguayans, led by the Marshal of the Republic Francisco Solano López, retaliated by attacking Mato Grosso on 15 December 1864 and later declared war against Argentina on 23 March 1865. The Blanco Government was toppled and replaced by a Colorado government under General Venancio Flores on 22 February 1865. Afterward, the Argentine Republic, the Empire of Brazil and the Republic of Uruguay signed the Secret Treaty of the Triple Alliance against the Paraguayan Government on 1 May 1865.

On 24 May 1866, the Battle of Tuyutí led to the loss of 6,000 men when a Paraguayan attack was repelled by the Allies. It was marked as the bloodiest battle in South America during the war.

The Paraguayans put up a ferocious resistance but ultimately lost in 1870 in the Battle of Cerro Corá, where Marshal Solano López refused to surrender and died in action. The real causes of this war, which remains the bloodiest international conflict in the history of The Americas, are still highly debatable.

Paraguay lost 25–33% of its territory to Argentina and Brazil, paid an enormous war debt, and sold large amounts of national properties to stabilize its internal budget. The worst consequence of the war was the catastrophic loss of population. At least 50% of Paraguayans died during the conflict, numbers to which it took many decades for the country to return. Of the disaster suffered by the Paraguayans at the outcome of the war, William D. Rubinstein wrote:

"The normal estimate is that of a Paraguayan population of somewhere between 450,000 and 900,000, only 220,000 survived the war, of whom only 28,000 were adult males."

During the pillaging of Asunción in 1869, the Imperial Brazilian Army packed up and transported the Paraguayan National Archives to Rio de Janeiro. Brazil's records from the war have remained classified. This has made Paraguayan history in the colonial and early national periods difficult to research and study.