The Passage of Humaitá (Portuguese: Passagem de Humaitá) was an operation of riverine warfare during the Paraguayan War − the most lethal in South American history − in which a force of six Imperial Brazilian Navy armoured vessels was ordered to dash past under the guns of the Paraguayan fortress of Humaitá. Some competent neutral observers had considered that the feat was very nearly impossible.
The purpose of the exercise was to stop the Paraguayans resupplying the fortress by river, and to provide the Empire of Brazil and its Allies with a much-needed propaganda victory. The attempt took place on 19 February 1868 and was successful – the attackers had hit upon the fortress' Achilles heel. It restored the reputation of the Brazilian navy and the Brazilian Empire's financial credit, and caused the Paraguayans to evacuate their capital Asunción. Some authors have considered that it was the turning point or culminating event of the war. The fortress, by then fully surrounded by Allied forces on land or blockaded by water, was captured on 25 July 1868.
Historical background
The Fortress of Humaitá was a defensive system on a bend in the River Paraguay, not far from its mouth. Long considered to be almost impregnable, it prevented enemy shipping ascending the river and hence invading the Republic of Paraguay. There may have been other routes by which the country could have been invaded, but they were considered more difficult still. From the main article on the Fortress of Humaitá it appears that Marshal-President López of Paraguay may have calculated that, because the fortress could not be bypassed except at ruinous expense, he could risk making war on the much more populous Brazil and Argentina.
The Paraguayan War (also known as the War of the Triple Alliance) of 1864–1870 was the most lethal in South American history, and − in terms of its relative mortality − very possibly the worst in modern history, anywhere. It actually commenced when López seized the Brazilian ship Marques de Olinda on her routine voyage up the River Paraguay to the Brazilian province of Mato Grosso, then sent military forces to invade the province itself. It developed further when he seized Argentine naval vessels moored in the port of Corrientes, north east Argentina. Thereafter López sent two further armies, one to invade the Argentine province of Corrientes along the River Paraná and the other to invade the Brazilian province of Rio Grande do Sul along the River Uruguay. On 1 May 1865 Brazil, Argentina and Uruguay signed the Treaty of the Triple Alliance by which they would not negotiate peace with Paraguay until the government of López had been deposed.
Further, for Brazil the river was the only practicable route to its interior province of Mato Grosso, which had been invaded by the forces of López earlier in the War. The war aims of the Allies (Brazil, Argentina and Uruguay) explicitly included the capture and destruction of Humaitá, and it was the prime military objective of the Allied high command.
The Allies' strategic problem
It took the Allies about a year to defeat the invading Paraguayan forces in the Brazilian province of Rio Grande do Sul and expel them from Argentina. According to their treaty they now had to neutralise Humaitá and depose López. In April 1866 the Allies crossed the River Paraná into southwestern Paraguay, occupying a small strip of territory. Their next strategic problem was how to proceed up the River Paraguay, getting past the Fortress of Humaitá.
Briefly, the fortress of Humaitá was built on a sharp concave bend in the river and comprised more than a mile of heavy artillery batteries atop a low cliff. The channel was only 200 yards wide, and ran within easy range of the batteries; a heavy chain boom could be raised to block the navigation and detain the shipping under the guns. 'Torpedoes' (improvised contact naval mines) could be released or anchored in the stream. On its landward side the fortress was protected either by impassable terrain or by 8 miles of trenches with 120 heavy guns and a garrison of 18,000 men. An attempt to capture one of its outworks by frontal attack failed disastrously at the Battle of Curupayty (22 September 1866); another frontal attack was out of the question.