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.

[22][23] From a naval perspective the heavy guns of the fortress might – in principle – be bypassed by the latest ironclads (armoured vessels), provided the chain boom could be cut and any 'torpedoes' avoided.

[15] Ammunition or other stores shipped from Rio de Janeiro, Brazil to the naval arsenal at Isla del Cerrito, near the mouth of the river Paraguay, had to travel an extensive water route.

According to the English-language Buenos Ayres Standard Experienced American and English and French naval officers, who had seen Humaitá, inspected the position, and gone through the batteries, all unanimously agreed on its extreme strength.

[81] It was the modern ironclad vessels of the Brazilian navy that had stood by and allowed the retiring Paraguayan army to escape across the River Paraná on rafts – together with 100,000 head of rustled cattle – without doing anything to stop them.

[94] Paraguay was fighting on her home territory; her artillerymen were good;[95] she could cast large guns at her foundries at Asunción and Ibicuy;[96] and foreign observers − including the Allies − were unanimous that her men were superbly brave fighters.

According to the diplomat Gregorio Benítes, who represented Paraguay abroad during the war: If Marshal López had not been precipitate in accepting the war, to which he was provoked by the Empire of Brazil, and had he given himself time enough to take delivery of the great monitors and armaments which he had ordered in Europe, among the last some 36 coastal artillery pieces made on the Krupp system, contracted for in Germany with the personal intervention of the undersigned, then not a single fresh-water ironclad would have been able to attack the aforementioned fortresses with success, if they had been armed with that powerful artillery.

In effect, whoever has any idea of the formidable power of Krupp artillery, will understand that such cannon being mounted in the batteries of Curuzú, Curupaity and Humaitá, the triple alliance would not have had one single ironclad which could venture under their fires, on pain of certain disaster.

Furthermore, López left it too late to order these weapons; so that the Baron of Itajubá the Brazilian accredited diplomat in Berlin, on finding out about the acquisition of those cannon on behalf of Paraguay, made use of his right under international law to require the detention of the said artillery.

Mitre was for once insistent, and there followed a correspondence in which, eventually, the Brazilian admiral agreed to attack the preliminary outwork of Curupayty – a few miles down the river – and, if he got past there, to assail Humaitá at a later date.

[121] The government newspaper El Semanario de Avisos y Conocimientors Útiles claimed a victory, writing: "The [Brazilian squadron is wounded in the heart, and we have one part of its ironclads prisoner between Curupaty and Humaitá, devoid of resources, without means of repair, and given over to the fury of our cannon; the rest will have to come to its aid, and follow the same road of shame and defeat".

From the main article on Inácio it appears that he was prone to depression; on 30 August he sent his subordinate commanders a questionnaire as to the feasibility of passing Humaitá; the questions suggested the answers he wished to receive;[135] for example:

Finally: in the present revolutionary circumstances of the Republics of the River Plate [meaning Argentina], knowing their feelings about the Empire of Brazil, is it prudent to risk the most important part of our Navy to a certain and inevitable ruin, without being convinced that it avoids a greater evil, or gives success to our cause and triumph to our arms?

From Rio steamers stopped to coal at Montevideo, Uruguay and then steamed non-stop to Corrientes, Argentina to drop off correspondence before proceeding to the naval arsenal of Cerrito near the mouth of the River Paraguay.

According to Artur Silveira da Mota, Admiral Inácio sank into inertia, rarely leaving his cabin aboard Brasil, instead writing picturesque articles for the weekly Semana Illustrada under the pen name Leva-Arriba (Get Up and Go), "a pseudonym which circumstances did not permit him to confirm by deeds".

Since the ironclads showed no sign of retreating below Curupaty, the Paraguayans had time to remove nearly all its heavy guns, convey them by land, and mount them at the next obstacle, Humaitá.

[165][166][167] By this time the Allied land forces had executed a strategic flanking manoeuvre, bypassing the Fortress complex on its east, eventually rejoining the bank of the River Paraguay at Tayí,[168] well to the north of Humaitá (see map).

[179][180] The squadron chosen to force the passage (19 February 1868) was as follows, and proceeded in this order:[181] At about 04:00 hours a signal rocket from Barroso announced that the first pair had successfully passed over the chain boom,[188] by now submerged in the river mud.

[195][196][197] It was still possible to be wounded by wooden splinters detached by the mechanical shock of an incoming cannonball − as in the days of sailing ships − and in the ensuing action Barroso had one such case, as did Alagôas.

Our naval officers have reported that the cast-iron projectiles impinging upon the armour, shivered into irregular fragments, which formed a hail of red-hot iron, and left the gun without a gunner to work it.

The battery men always knew when a ball struck the plates at night, by the bright flash which followed the shock[65]As Alagôas steamed past Timbó, 20 canoes started from the shore full of armed Paraguayans intent on boarding her.

In a special edition, the Irish-edited[208] Buenos Ayres Standard[209] said: The cannon of the Brazilian iron-clads, as they forced the passage of Humaitá, has[210] re-echoed over this continent, and will reverberate through Europe.

[211] In Rio de Janeiro the British minister George Buckley Mathew advised the Foreign Secretary Lord Stanley: [It is] a feat that will find its place in the annals of the military and naval science of the age, and that vindicates a high standard for the courage and the discipline of the Brazilian navy.

[213] The Brazil and River Plate mails arrived at Southampton on 6 April carrying the South American newspapers; the next day The Times of London printed extensive verbatim extracts describing the passage of Humaitá.

An article in the consistently pro-Paraguayan[216] Colburn's United Service Magazine and Naval and Military Journal, of London, admitted that the Brazilian navy had achieved what some excellent judges of such matters considered very nearly an impossibility.

In his official report to Parliament the British consul in Rio de Janeiro remarked that the Brazilian paper currency and the price of government bonds had dropped to half their prewar value and were still falling.

A rise in the river enabled the squadron to pass Humaitá out of range [sic],[223] and turned the tide of the war, raising the rate of exchange by shewing a probability of an early cessation of the hostilities.

Military, because it was the principal strategic objective of the Brazilian squadron; political, because it sanctioned the right, traditionally upheld by Brazil, of free access by her shipping to the rivers of which she was a riparian superior.

[236] On the other hand in their jointly-written introduction to I Die with My Country: Perspectives on the Paraguayan War, 1864–1870 (2004) Professors Whigham and Kraay wrote that the passage of Humaitá was a feat of limited military significance.