The siege of Humaitá was a military operation in which the Triple Alliance (Brazil, Argentina and Uruguay) flanked, besieged and captured the Fortress of Humaitá, a Paraguayan stronghold that was referred to as the Gibraltar of South America. It fell on 26 July 1868. It can be considered the key event of the Paraguayan War since the fortress had frustrated the allied advance into Paraguay for more than two years. However it did not surrender as the defenders escaped, most of them to fight another day.

The allies were severely criticised for the time it took them to take the installation, whose strength was belittled. But they were essentially a pre-professional army fighting a long way from home against an unaccustomed defence tactic: artillery in prepared, entrenched positions firing a hail of anti-personnel shot. Further, they were battling in unprecedented terrain — most of it impassible, all of it unmapped — in the wetlands of southern Paraguay. It gave the Paraguayans, who fought with exemplary courage despite being abysmally supplied, a huge advantage.

Moreover the allies, who at first had attempted to take the fortress by frontal attack from the Paraguay River, had instead suffered a catastrophic defeat; this had disorganised and demoralised their forces. Restoring their military effectiveness was a task that fell on the shoulders of the new commanding general the Marquess of Caxias. For political reasons he could not risk another disaster.

Context and importance Brief recapitulation The Paraguayan War was "the bloodiest inter-state war anywhere in the world between the end of the Napoleonic Wars in 1815 and the outbreak of the First World War in 1914", and far and away the most lethal in the history of South America. Yet it is little known outside that continent. It had three phases: Paraguay declares war on Brazil and Argentina — countries vastly more populous than itself — invading three of their provinces. The two countries, plus Uruguay, sign the Treaty of the Triple Alliance, vowing never to lay down their arms until they have toppled Paraguay's leader Francisco Solano López. The Allies invade Paraguay, and at length capture its capital Asunción; Guerrilla warfare and the pursuit of López, who is finally killed by Brazilian cavalry after refusing to surrender.

Its key event was the capture of the Fortress of Humaitá, which was the gateway to Paraguay, and had held up the allied advance for over two years.

The maps show why the fortress was the key to Paraguay. The country is landlocked, but can be accessed from the sea; one must sail up two of South America's great rivers in succession: The River Paraná, which traverses many miles of Argentine territory. (A small portion of the Paraná is visible at the bottom of the Context map, marking the boundary between Argentina and Paraguay.) The River Paraguay, the country's principal highway,There were a few roads () and a number of rough trails. and after which it is named.

As shown, Humaitá was a few miles up the river Paraguay. At first it was just a modest guardia or lookout point manned by a few soldiers.

In the 1850s there had been two separate incidents in which Paraguay had been threatened by the naval flotillas of irate foreign powers, namely Brazil and the United States.A Brazilian flotilla of 1855 demanding free access to the Mato Grosso (), and an 1859 U.S. expedition demanding reparations for an outrage to the American flag. It was the paddle steamer USS Water Witch which first showed how easy it was to navigate the river Paraguay. See the article Paraguay expedition and the sources cited there. It became apparent that warships, though based in the Atlantic Ocean, could steam up into Paraguay quite rapidly.Sailing ships used to take 6 months to get to Humaitá; steamships could easily do it in a fortnight: . Thus, the transition from sail to steam made the Paraguayan government realise that, far from being secure in the heart of South America, it was vulnerable.

Accordingly, the Paraguayans had greatly strengthened the fortress of Humaitá. Its purpose was to bar the way in to Paraguay. Commanding a hairpin bend in the river, it had a formidable reputation.

Brazilian frustration For the Brazilians the fortress was doubly vexing. It blocked the river route to their province of Mato Grosso, which lay hundreds of miles further upstream, and had been occupied by Paraguay since the war began. It was by far the most convenient means of access to their territory, a land about the size of Germany. Unless they got past Humaitá there seemed little prospect of recovering it.Because the overland route was difficult and dangerous. The Brazilians did try it nevertheless, but with disastrous results: .

Attempt to take fortress by storm

Hoping for a quick result, the Allies tried to take the fortress by frontal assault from the river Paraguay.

On 22 September 1866 they landed troops on the edge of the only substantial piece of firm ground, a meadow marked M on the map Terrain. To attack in that place was a rational decision in itself, for they had hit upon a weak spot, and they might well have prevailed, greatly shortening the war. But their commands were divided, they dithered and delayed, and the Paraguayans built a defensive earthwork in the nick of time.

Armed with artillery firing a hail of grapeshot and canister, it destroyed the allied attack; many Argentine and Brazilian soldiers, advancing over open ground, were killed.

The Paraguayans, staying behind their defences, had few losses.

This was the battle of Curupayty, the allies' first major setback, and their worst defeat of the war.

Summary: situation in October 1866 Thus, the allied armies, having landed in Paraguay, controlled only 9 square miles (2,330 hectares) of its territory: inhospitable wetlands. Their advance was stymied by this seemingly invincible fortress; and, as will now appear. they were thoroughly demoralised.

Aftermath of the Battle of Curupayty The allied advance into Paraguay came to a complete halt after the disaster of Curupayty.

Command changes in the Allied army

The defeat caused dissension among the allied commanders and no one knew what to do next.

After the battle, Joaquim Marques Lisboa, the Viscount of Tamandaré and commander of the Imperial Brazilian Navy, Polidoro Jordão, commander of the 1st Brazilian Army Corps, and Venancio Flores,Flores had already planned to retire and head back to his country two weeks earlier, but delayed his departure in order to take part in the battle: . commander of the Uruguayan army, left their positions. Tamandaré was replaced by Joaquim José Inácio and Flores was replaced by Enrique Castro; according to historian Thomas Whigham, Uruguayan participation in the war was now reduced to a "token force only nominally Uruguayan in composition".

The question of who should lead the Imperial forces remained however. In Brazil, emperor Pedro II was determined to continue the war despite Curupayty; the new cabinet, headed by prime minister Zacarias de Góis e Vasconcelos, of the Progressive League, which rose to power seven weeks before the battle, was tasked with continuing the war.

The emperor then nominated an experienced and prestigious general to lead the Imperial forces in Paraguay: Luís Alves de Lima e Silva, then Marquess of Caxias, by a decree on 10 October 1866.The name of Caxias had been considered to lead the Brazilian forces in Paraguay already in February 1865, even being contacted; however, due to his personal disagreements with Ângelo Moniz da Silva Ferraz, the Minister of War, Caxias was not chosen. Silva Ferraz was passed over at his own request that same day and replaced by João Lustosa da Cunha Paranaguá, who was Minister of Justice: . The decree did not give Caxias command of the Brazilian navy; thus on paper it did not unify command of the Brazilian land and naval forces; despite this, the navy also fell under Caxias' command, and would no longer operate separately as it had done since the start of the war.According to Article 3 of the Treaty of the Triple Alliance, the navy was independent from the land forces: . There was a concern on the part of the Brazilian government not to leave the navy under Argentine president Bartolomé Mitre's command.

Caxias took his first steps by creating the 3rd Army Corps in Rio Grande do Sul already on 18 October 1866, while still in Rio de Janeiro. He departed aboard the steamer Arinos on 29 October 1866, stopping at Montevideo on 2 November, Buenos Aires on 6 November, Rosario on 9 November and Corrientes on 14 November, finally reaching the allied camp in Tuyutí on 18 November. Upon his arrival, he issued his Order of the Day No.1, in which he stated to the troops that "if I didn't already know you, I would recommend you valour; but in the countless battles that have taken place to date, you have given ample proof of this military virtue"; the next day he assumed unified command of the 1st and 2nd Brazilian Army Corps.

Epidemics and overall state of the army

At that point several diseases had spread in the battlefield, but a new cholera outbreak proved worst, killing or incapacitating thousands of soldiers;Whigham states that by the end of April 1866 thirteen thousand Brazilian soldiers had been incapacitated by the disease: . the cholera epidemic became so serious that the Allied commanders tried to disguise it from both the Paraguayans and the civilian population in their respective countries, even prohibiting the use of the word "cholera" in official communications. By March 1867 the epidemic had spread to Buenos Aires and Montevideo. Less than a year later, on 2 January 1868, the illness was to kill Argentine vice-president Marcos Paz.

The military camps did not follow basic hygiene practices and were filthy and disorganised. The soldiers often drank contaminated water, which they got by excavacating the nearby sandbanks; the water was contaminated by dead bodies that were frequently buried near the water sources. In order to cool a demijohn of water, the soldiers would dig a hole in the ground and bury it. , a Brazilian soldier, later recalled, after having ordered a hole to be dug in his tent, that "as soon as the comrade had reached the bottom, we felt the characteristic smell of death. One more hoe hit and a rotten skull appeared. He plugged the hole and dug another one ahead".

The sanitary conditions were not the only issue, for the organization and morale of the allied army were also abysmal. Referring to the state of the 1st and 2nd Brazilian Army Corps, Caxias later stated that "they seemed to belong to two different nations", as each had their own administration, accounting, payment and promotion criteria. This was also a problem in other units, especially the corps of Voluntários da Pátria (patriot volunteers).

Another issue was the general lack of materiel: the cavalry was short of horses and those it had were poorly fed;One of the reasons for the lack of horses was precisely their poor feeding, as they were fed improper vegetation taken from the local wetlands, which weakened and killed many of them: . the 2nd Army Corps, for example, was completely dismounted; many soldiers had traded their uniforms and walked half-naked or barefoot; the Minié rifles used by the infantry needed a ramrod to function, many of which had been damaged; the artillery lacked heavy calibre guns, something that some observers pointed out as one of the causes for the allied defeat at Curupayty; there were also no ox carts or oxen to transport goods and troops.

Federalist rebellions in Argentina