The Paraguayan War began with ambition. President Francisco Solano López, fearing Brazilian expansion, sent two great columns south through Argentine territory in 1865 - one to seize the city of Corrientes, the other to strike at Brazilian lands along the Uruguay River. For a moment the plan worked. The Paraguayans took Corrientes; General Estigarribia's column crossed into Brazil and captured town after town. But the Imperial Brazilian fleet shattered the Paraguayan squadron at the naval Battle of Riachuelo, severing the rivers that were supposed to carry supplies and reinforcements. The invading columns were suddenly cut off from each other and from home, deep in hostile country, with the allied armies of Argentina, Brazil, and Uruguay converging from every direction.

Estigarribia detached Duarte with roughly three thousand men to guard the Uruguay River, then marched on without him. When the allied general Venancio Flores joined forces with Wenceslao Paunero's troops on August 13, their combined army swelled to twelve thousand - nearly four times Duarte's strength. Duarte sent word to Estigarribia begging for support. The reply was so dismissive that, insulted, Duarte resolved to give battle anyway. He pulled back from Paso de los Libres and took up positions on the banks of the Yatay stream. Both the Yatay and the Uruguay had recently overflowed, turning the battlefield to mud and water. The marsh shielded his men from a frontal charge, but the stream at their backs meant there could be no retreat. Defeat, Duarte knew, would mean annihilation.

The battle opened at ten in the morning when allied infantry under Leon de Pallejas struck the Paraguayan line. Duarte seized the moment and counterattacked with nearly all his cavalry, inflicting hundreds of casualties and driving the attackers back. For two hours the fighting was waged on horseback alone, sabers and lances in the flooded fields, while Duarte's outnumbered horsemen held. But numbers tell. When the allied infantry finally pressed forward in force, the Paraguayans fought with a tenacity that impressed even their enemies. In a last desperate charge Duarte's horse was shot from under him. Paunero himself rode up and demanded his surrender. There was nothing left to do but agree.

Of the roughly three thousand who had stood on that riverbank, the Paraguayans counted seventeen hundred dead, three hundred wounded, and twelve hundred taken prisoner. A few hundred swam the Uruguay River to escape. These were not statistics to the men who lived it - they were conscripts and volunteers who had marched a thousand miles from home and were cut down in a swamp far from anyone who would mourn them. Among the captives, Flores found Uruguayan exiles of the rival Blanco party and had them executed as traitors, a grim coda to a grim day. A month later Estigarribia surrendered at Uruguaiana, and the war moved north into Paraguay, where it would grind on until 1870 and consume a staggering share of that nation's men. Yatay was only the beginning.

There is no grand monument on the field at Yatay - only the quiet bend of a stream near Paso de los Libres, the Uruguay River sliding past toward the Atlantic, and the flat green country of Corrientes that floods still when the rains come. The battle survives mostly in memory and in art. Cándido López, the young lieutenant who sketched the campaign, lost his right arm to a grenade at a later battle, taught himself to paint with his left hand, and spent his life rendering the war he had seen - including the wounded prisoners of Yatay. Far to the south, a street in the Buenos Aires neighborhood of Caballito still bears the battle's name, a small thread connecting a crowded modern city to a flooded field where three thousand men refused to run.