Humaitá
The Allied failure in Curupaity led to ten months of restless peace between the two opponents. Nonetheless, skirmishing, sniping and daily bombardment kept the armies on alert.
Located on the Passo de La Pátria side of Paraguay River, Humaitá Fortress successfully prevented the Allied Fleet from reaching Assuncíon. The fortified system of defense, which Humaitá was the strongest point, lay from the shoreline of Paraguay River to the openings and passages near the swamps of Estero Bellaco. It consisted of lines of trenches, strongholds and a moat on Curupaity side of the river. Near the shore great boxes of stones were sank to prevail land operations. A British diplomat who visited Humaitá in 1867 noted:
"The riverside batteries of Humaitá at present mount only 46 guns, namely one 80-pounder, four 68-pounders, eight 32-pounders;the rest are of different calibers. The battery of Curupaity towards the river mounts thirty 32-pounders. The center is defended by about a hundred guns. On the left are 117 guns, including four 68-pounders, one 40-pounder rifled Whitworth(...), one 13-inch mortar, fourteen 32-pounders and many rifled 12-pounders. Humaitá on the land  side is protected by three lines of earthworks, on the innermost of which 87 guns are mounted. Total on the left, 204 guns. The grand total is, therefore, 380 guns"
Only by frontal assault or by the fleet's passage of the fortress the Allies could take the system of trenches, referred by them as the Quadrilátero. By November, 1866, the Allied Fleet began incessantly bombarding of the Quadrilátero. The major problem of the navy was the shallow waters around Humaitá and Curupaity. This kept the ships at some distance making difficult an effective fire support. Thus, the cannonballs themselves made little harm to the Paraguayan defenses. They were, however, good to maintain the morale of the Allies after Curupaity. In January, 1867, however, a shell struck General Díaz' canoe in a reconnaissance duty. He died almost a month later on February 7.
With Paraná River blocked since June, 1865, the Paraguayan Army was in great need of supplies and ammunition. López gave orders no to fire a single shot unless a worthy target came into range of  cannons. After Curupaity, the army relied at most on re-used shells launched by the enemy fleet or captured weapons.
Along the Quadrilátero perimeter, López had at disposal 20,000 men; some 15,000 infantry, 3,500 cavalry and 1,500 artillerymen. This army, nevertheless, represented the last manpower resources available. Most of the best units had vanished. In their place there were only young boys and old men.
The only advantage his troops had was the terrain. It was crossed by innumerable streams and covered by  broad swamps and thicket. The enemies would have to pay a high price for each meter they gain.
Marquis of Caxias, commander-in-chief of the Allies since November 1866
Meantime, the Allies were facing  their own problems.
The new commander-in-chief, General Luís Alves de Lima e Silva, Marquis of Caxias, encountered the Alliance Forces in chaotically circumstances. By the time he arrived in Passo de La Pátria in November, 1866, the Imperial Army alone had suffered 10,000 casualties caused by cholera and looseness since April, 1866. Others 7,000 to 8,000 were killed or wounded in combat. Besides, the Argentinean and Uruguayan armies were reduced in the war zone. Finally, the navy was in need of new units in order to face Humaitá fire power.
Caxias decided for a continuous reinforcement of the army by a steady flow of recruits and acquisition of new rifles and artillery pieces. His plan was simple: he would keep the bombardment of Humaitá as the forces at his disposal grew day after day. Then, he would launch a slow but enduring movement to strangle López' position. The Allies would concentrate on defeating the lines of trenches one by one. Once Humaitá was encircled and weakened it would be attacked. It would take months before the army ranks grew and the navy received new armored monitors, but Caxias was a patient commander. He knew that a frontal assault would result in defeat.
By mid-July, 1867 the force reached its climax. The Triple Alliance Force grew to almost 45,000 troops, of which 40,000 were Brazilians, some 4,000 were Argentineans and a few hundreds were Uruguayans.
Caxias first planned a flank maneuver to encircle the Quadrilátero. The I Corps of the Imperial Army would joined the newly formed III Corps to head to San Solano, northward of Humaitá. The march involved the passage though difficult terrain, where sometime a man could barely walk with water around the chest. Lieutenant-General Osório was given command of the the III Corps and would lead the march. Meanwhile, the II Corps would remain at Tuyuty as reserve force and also to protect the lines along Estero Bellaco.
At day break of July 22, Osório began moving. By nightfall he reached San Solano from where the tower of Humaitá Church could be seen. A battery of guns was positioned there. He left some men on San Solano and moved to join the I Corps again to assault the Upper Paraná, cleaning the area from any Paraguayan presence.
On August 18, the fleet succeeded in forcing the passage at Curupaity. The Paraguayans withdrew to inner positions inside the Quadrilátero.
On November 2, the siege by land is completed with the fall of Tayí, a small fortified position.
General Osório leading the III Corps in a movement to encircle Humaita.  Brazilian Army Archives
The gradual encirclement forced the cut of communication between Humaitá and Assuncíon. The pressure on the Paraguayan lines led López to plan a swift assault on Tuyuty. He believed that this maneuver at the most improbable point of the front could succeed in breaking the siege, bringing some relief to his troops.
General Barrios was chosen to command the attack. He also had orders to bring as many prisoners he could along with supplies and weapons. On the morning of November 3, he led 8,000 troops in the assault. By this time, the action took the Allies in amazing surprise. The first units Barrios' men met offered little resistance. They took possession of whatever they could and soon the attack became a looting. At this time the II Corps, under Porto Alegre, recovered from surprise. In the fray he gathered five battalions to give combat to the assailants. Leading from ahead, Porto Alegre had two horses killed from under him. He stopped just when the wounds he received unable him to continue the fight.
When the Paraguayans withdrew they brought with them 14 cannons and 250 prisoners.
The attack although inconclusive, showed the Allies that the Paraguayans were still capable to make offensive movements. They also had a moral victory by making prisoners and bringing supply and weapons to their lines. Notwithstanding, the attack did not achieve its strategic aim: the siege was not lifted. On the contrary, after the Second Tuyuty, Caxias' determination to tight the siege increased. The bombardment of Humaitá proceeded while small movements gained more terrain.
With some units of the I Corps Caxias steered ahead to La Cierva redoubt only two miles north of Humaitá, on February 18, 1868. After a fierce resistance, the Paraguayans, much outnumbered, retreated and by nightfall the place was under firm control of Caxias' men. In this fight, the Paraguayan losses amounted to 150 casualties while the I Corps losses totaled almost 600.
At that same time, the fleet forced the passage over Humaitá. despite the heavy fire the armored ships Barroso, Tamandaré and Brasil were successful in reaching a position above the fortress. The way to Assuncíon lay open. The ships steamed north and by the morning of the 24th Assuncíon was briefly bombarded. This fact had a immense impact over Paraguay's leadership. Some of them became conscious that the war was lost.
On the night of March 1, López launched a desperate attack on the Brazilian warships anchored above Humaita. He gathered a fleet of canoes and sent it against the armored monitors Herval, Barroso and Brasil. The failure costed the Paraguayan many lives while the Brazilians sustained only a dead and some wounded. Thus, López decided for the evacuation of Humaitá. During two days, the 2nd and 3rd, mainly by night, he led the bulk of his forces to the Chaco. According to some sources, he fled Humaitá with a force of 10,000 to 12,000 strong, leaving a garrison of some 3,000 under Colonel Paulino Alén to meet the enemy. Alén was ordered to resist until the last man.
Nevertheless, on the 21th, the last defensible stronghold around Humaitá fell to the II Corps. The next day, the remaining Paraguayan forces inside the Quadrilátero sought shelter in the fortress.
Vice-Admiral Joaquim José Ignácio,
new commander of the Imperial Fleet
Despite the growing pressure, the defenders of Humaitá continued to fight. A road opened in the Chaco from where they received some aid, fell to the enemy. Colonel Francisco Martinez, the second-in-command in Humaitá were facing growing difficulties. After Colonel Alén's attempt of suicide he assumed command of Humaitá with little food, scarce ammunition, sickness decimating his troops and no hope of reinforcements. Notwithstanding, on July 16, he resisted an assault by the Brazilian troops. From the wall ,the defenders shot deadly volleys on the assailants. The 3rd Corps units pulled back. The Paraguayans suffered in the attack 261 casualties while the enemy sustained 1,031.The Paraguayans still persisted.
Caxias was convinced that the Paraguayans could not maintain the position for much time and decided to keep the pressure. On the night of the 24th, Martinez fled the fortress crossing to the Chaco. The allies entered Humaitá the following day.
This did not mean the end of the fight in the outskirts of the fortress. Caxias ordered a final assault on Andaí, the last position guarded by Martinez in the Chaco. On the 28th three batallions were sent to expel the Paraguayans from that position, forcing them to surrender. Martinez, however, held the position.
On August 5, however,  convinced of the hopelessness of his situation Martinez decided to surrender. The remaining 1,300 officers and men on his command were imprisoned. The struggle for Humaitá was finally over. Both sides were mauled terribly.
The Allies estimates of  casualties are suspicious low for the entire campaign. Brazilian sources put the amount of Allies casualties around 8,065 since September, 1866. A more reliable figure, however, can be put in some 10,000 since July , 1867. The Paraguayans, as far as we know, suffered 4,100 casualties among dead, wounded and captured.
The fall of Humaitá removed all obstacles from the path of Allied occupation of Assuncíon. from this point the war became disastrous to Paraguay. Pushing the fight to the inner villages of Paraguay, the Allies and the Paraguayan Army brought not only war but also cholera to the countryside  inhabitants.
Dezembrada