The Fortress of Humaita was built on the strategic bend of the Paraguay River that would force ships to steam head first into artillery fire. The Allied Command assumed that because of that and the chains installed, the fortress as impassible.
Encirclement
On 1 August 1867, the Argentine general Bartolomé Mitre commanded the Brazilian imperial fleet to secure a passage through Curupaiti and Humaitá. On 15 August, two divisions of five battleships passed Curupaiti without incident but the artillery fire forced them to stop at Humaitá. This news caused conflict in the Allied high command. The Brazilian commanders were determined that attacking the fortress from the river would be futile and withdrew their fleet, pending a land-based attack that began on 18 August. From Tuyucuê, the Allies headed north and took the villages of São Solano, Tayi and finally laid siege to Humaitá itself, isolating it from Asunción by November 1867. On 19 February 1868, Marshal Caxias and Vice Admiral Baron de Inhauma ordered the fleet up the River Paraguay past Humaitá. There were few casualties and minimal damage caused to any of the ships. On 24 February, the Bahia, Barroso and Rio Grande do Sul shelled Asunción, which had been evacuated earlier. Paraguayan PresidentFrancisco Solano López decided to evacuate Curupayti and Humaita. He crossed the Paraguay River to the Chaco side on 3 March 1868. Solano López left Colonel Francisco Martinez in charge of a force of 3,000 men and 200 cannons. General Argollo attacked Sauce on 21 March, resulting in the Paraguayans retreating to Paso Pacu. Curupayti was abandoned the next day. The Bahia, Rio Grande and Para bombarded Humaitá on 23 and 24 March 1868. At the end of April, Allied forces had troops on the Chaco side of the river.
Battle of Acayuazú
The Allied army advanced on 16 July 1868, when it appeared Curupayti and Humaitá were abandoned. General Osório and 6,000 troops led an attack on the northeast side of Humaitá, unaware of the 46 concealed Paraguayan guns and over 2,000 men under the command of Colonel Pedro Hermosa. At the command of "Muerte a los cambas", the Brazilians retreated. The Brazilian casualties consisted of 279 dead, 754 wounded and 100 captured, while the Paraguayan casualties were 89 killed and 104 wounded. General Rivas ordered an attack on the Paraguayan Cora redoubt on 18 July,. An allied force of Argentinian and Brazilian infantry men were ambushed by a force led by Colonel Caballero. The Argentinian losses amounted to 90 killed, 87 wounded and 35 captured, while the Brazilians had 67 killed, 221 wounded and 2 captured. The Paraguayans suffered 120 casualties.
Evacuation of Humaitá
Col. Martinez asked Solano López for permission to start evacuating Humaitá on 19 July. Solano López ordered Martinez to hold out five more days, but the first wave of Paraguayan withdrawal started on the 24th with 1,200 men. The remainder left on the 25th after spiking their guns. The Allied force entered Humaitá ten hours later.
Aftermath
Col. Martinez, along with 1,228 men, 96 officers, and women and children, were caught trying to cross Laguna Vera on 5 August 1868 where he finally surrendered. Lopez branded Martinez a traitor and sought vengeance by murdering his wife. The allies captured 146 iron guns and 36, but most were unserviceable. A few men, Col. Alen among them, managed to escape the encirclement and make it back to Paraguayan lines via the jungle. However Col. Alen wasn't received with much adulation and was instead arrested for desertion.
