The Paraguayan War pitted the landlocked nation of Paraguay against the combined might of Brazil, Argentina, and Uruguay. In its opening phase, President Francisco Solano López sent his armies south and east, deep into enemy territory. On 5 August 1865, a column under Lieutenant Colonel Antonio de la Cruz Estigarribia took Uruguaiana without firing a shot. The town surrendered, and the Paraguayans occupied it with around ten thousand men, fortifying their position with an abattis, a defensive tangle of sharpened branches. This was the high-water mark of the invasion. Uruguaiana marks the southernmost point Paraguayan forces ever reached. Everything that followed was retreat. By the time the noose closed, hunger and desertion had already thinned that garrison to roughly half its strength.

The army that gathered to retake Uruguaiana was a fractious one. Brazil, Argentina, and Uruguay had signed the Treaty of the Triple Alliance, but their commanders distrusted one another deeply. The Count of Porto Alegre nursed an open contempt for his Hispanic-American allies. When the Uruguayan president, Venancio Flores, boasted that his men could defeat the Paraguayans alone, the Brazilian officers mocked him to his face. Argentina's president, Bartolomé Mitre, claimed command of the whole force, and Porto Alegre flatly refused to recognize it, reminding Mitre that on Brazilian soil a Brazilian would lead. The squabbling stopped only when the emperor himself arrived and quietly divided the army into three, one part for each nation's pride.

Pedro II reached the siege lines on 11 September. It was an extraordinary thing for a head of state to do, and he did not come alone. The presidents of Argentina and Uruguay were there, along with the Count d'Eu and Admiral Tamandaré, who commanded the Brazilian fleet on the river. Roughly seventeen thousand allied soldiers and fifty-four guns now encircled Estigarribia's shrinking, hungry garrison. The Paraguayan commander knew the math. When the end came, he made one condition: he would surrender only to the Brazilian Emperor, not to the republicans of Argentina and Uruguay, whom he did not trust. On 18 September, with nothing left to eat but sugar, he laid down his arms.

The surrender broke the back of Paraguay's offensive. López, recognizing that the war had turned, abandoned the Argentine province of Corrientes and pulled back to defend his own frontier. The withdrawal was vast and methodical: General Resquín drove more than 100,000 head of cattle and other livestock across the Paraguay River at Paso de Patria over four days in late autumn. The men who had marched south in triumph now retreated north, herding the spoils of an invasion that had ended in capitulation. The war itself would grind on for five more brutal years and become one of the deadliest conflicts in the history of South America, but the moment captured here, on a quiet stretch of riverbank, was the moment the tide turned. After Uruguaiana, Paraguay was never again on the attack.