At a very short distance, they found the Paraguayan defensive device made up of four fortifications, the so-called "Cuadrilátero", which obstructed the passage to Asunción both by land and by the Paraguay River.

Casualties on both sides were even higher from disease than from battle due to a cholera epidemic which was added to the appalling food and sanitary conditions.

From the Paraguayan point of view, the breakdown of this balance endangered its economic autonomy and hindered its efforts to prevent the territories in conflict from being annexed by their powerful neighbors.

Given the Argentine refusal, López declared war on Argentina on March 19,[8] and began the Corrientes campaign, from there, to Rio Grande do Sul.

The allied army concentrated in the city of Corrientes, where it was harassed by small Paraguayan incursions, which, although achieving a victory in the Battle of Pehuajó, failed to prevent the organization of the invasion of Paraguay.

Before starting the invasion, the Brazilian war fleet, supported only partially by the Argentine Navy, explored the enemy coasts only from a short distance from the point called "Tres Bocas", that is, the confluence of the Paraguay River with the Paraná.

The first man to make landfall was Brazilian general Manuel Luís Osório, then Baron of Erval, commander of the imperial forces, accompanied by a scant escort.

In doing so he unwisely exposed his troops, but López made a major mistake as instead of waiting for the invaders in their defensive lines, the Paraguayan forces attacked the allies at Estero Bellaco.

However, the Paraguayans managed to stop the advance in the open field at Tuyutí, where the invading forces limited themselves to waiting to be attacked to defend themselves.

The Humaitá Fortress was the most powerful and was located on high ravines, facing a very pronounced bend in the river, which forced ships to pass slowly and carefully under enemy fire.

With the allied army setting camp in the potrero of Tuyutí, López responded with a combined attack with most of his available troops, divided into four columns.

The plan could have succeeded in conditions of numerical superiority, but the Paraguayan chief assigned to the operation little more than half the troops of the allied army he had to face.

[12] Despite the great victory obtained, general Mitre continued to simply wait, giving López time to gather new contingents of soldiers.

The prevailing bad weather gave the Paraguayans the opportunity to reinforce the defenses, and also forced the attackers to fight through flooded marshes.

The Brazilian fleet, under the command of the Viscount of Tamandaré had promised to destroy the Paraguayan fortifications with its artillery from the Paraguay River, but the bombardment was carried out very ineffectively.

On 22 September 1866, the battle of Curupayty or Curupaytí took place, in which the attack of the allied forces was completely frustrated by the Paraguayan troops under the command of José E. Díaz.

A peace attempt, mediated by the United States ambassadors in Asunción and Buenos Aires, failed due to the double refusal of López and Pedro II of Brazil.

[24] Finally at the end of July, the Brazilian troops, commanded by Luís Alves de Lima e Silva, then Marquis of Caxias, left Tuyutí for the Tuyú Cué fortress, which was captured without a fight on the last day of that month.

[12] While Mitre assumed command again, a Brazilian naval squadron overcame the Curupayty cannons, but remained anchored between the fortress and Humaitá for six months, forcing the construction of a railway line through the Chaco to supply it.

He managed to cut López's telegraphic communications at several points, and recognized the route to Tahy and a place that he mistakenly called Cierva Redoubt.

Despite the inaccuracy of some of his reports, other data obtained was key: the enormous number of fortifications built by López to the south and southeast of Humaitá, the "Cuadrilátero" couldn't be attacked without serious losses.

[27] On 2 January 1868, Argentine Vice President Marcos Paz died in Buenos Aires from cholera and Mitre definitively abandoned the front.

It was the opportunity that the Imperial Brazilian Navy had waited for almost two years with the squadron reinforced by three monitors that were small and fully armed being ideal for the type of maneuvers that Humaitá's position required.

On February 19, after a heavy artillery exchange, some Brazilian ironclads were able to cross in front of the Humaitá fortress and three days later, two of them briefly bombarded Asunción, the city was then evacuated.

The Marquis of Caxias sent a division commanded by general Manuel Luís Osório to capture the fortress, but this attack was repelled on July 16 with more than a thousand Brazilian casualties against less than a hundred Paraguayans dead.

Due to the Brazilian naval advance, President López gave up defending the Tebicuary River line, installing a defensive front much closer to Asunción, on the Pikysyry stream.