On 9 July 1868, at 11:50 pm, 270 Paraguayan soldiers, known as bogavantes [pt], attacked two Brazilian battleships, Barroso and Rio Grande, anchored near the Tagy ravine, located on the left bank of the Paraguay River.

[2] After the passage, Humaitá became unreachable by the river, which forced the Paraguayans to build a road in the Chaco that linked the fortress to the fort of Timbó [pt] so that supplies were not cut.

However, the constant bombardment by Brazilian ironclads and the siege mounted by allied forces on land contributed to the Paraguayan decision to abandon Humaitá.

[3] The Paraguayans again attempted an assault, this time against two battleships, Cabral and Lima Barros, on 2 March, but once again failed after being repelled by the ships' crews and support from other vessels.

[1][4] These last Paraguayan actions led the Brazilian naval command to suspect that a new boarding attempt could occur against ships that were ahead of Humaitá.

Indeed, several Paraguayan prisoners, including officers, reported that Solano López was training soldiers from various divisions of the army and navy, known as bogavantes, to board and seize imperial battleships.

The other five battleships that had passed Humaitá began to bomb this position and Timbó, in addition to serving to transport troops and supplies for the allied forces in the Chaco and also in Tagy.

The ironclad Barroso, commanded by Artur Silveira da Mota, later Baron of Jaceguai, had a little more than one meter of freeboard, which decreased even further when the ship had to use its cannons, since about half of the edge was removable, reaching just over 60 centimeters the distance between the main floor and the water line.

The assailants jumped on the battleship's deck armed with hand grenades, spears, pikes, hatchets and tubes loaded with asphyxiating materials.

[13] The commander of Barroso decided to use the forward guns of the battleship only when a considerable number of bogavantes occupied the deck, since after firing the pieces needed men outside for the reloading work, that is, they would hardly be used again.

At one point in the engagement, thirty Paraguayans crowded the bow and a single shot from the 120-caliber cannon "was enough to sweep the deck, where only dead bodies or severely wounded remained."

[14] Despite the bogavantes' efforts, Barroso managed to move towards the cover of the army's batteries and the Rio Grande monitor, with a crew consisting of 43 men and officers,[15] which was already strafing the survivors in the water with its propeller.

At that moment, the captain of the Rio Grande, Antônio Joaquim, and some auxiliary soldiers, tried to prevent the ship from being boarded, an act that resulted in the disappearance of the commander[14] and injuries to the sailors.

The commander of the position, brigadier João Manuel Mena Barreto, placed the 40th Fatherland Volunteer Corps on the river bank, in line, and opened fire on the assailants.

The attempted assault against the vessels failed and the Paraguayan survivors tried to swim to the banks of the river, at which point Mena Barreto ordered shrapnel artillery to be fired on them.

Recklessly, the infantry there confused the crew with Paraguayans still trying to attack them and fired a volley, injuring four sailors and killing two others, being the only deaths on the ship.

The confusion appears to be without justification, since at the time there was "light that night due to the effect of the moon", according to the Artur Silveira da Mota's account.

One of the Paraguayan prisoners that took part in the assault indicated the defensive positions in the Tebicuary region, resulting in the transposition of three battleships on them and subsequent bombing of the bogavantes training site.

Even at a disadvantage, the Paraguayans offered fierce combat in canoea and only surrendered with the arrival of Imperial army troops garrisoned at Timbó, which took place on 5 August.