Colonel José Félix Estigarribia, the Paraguayan field commander, had spent weeks preparing a double envelopment, the maneuver every general studies and few ever pull off. With a force of some seventeen thousand men drawn from several divisions, he kept Bolivian attention fixed forward while Paraguayan columns swung wide around both flanks. Quietly, deliberately, they closed behind the enemy near Campo Vía, the dry creek bed whose name had already passed into the war's vocabulary that summer. By the time the Bolivian command grasped what was happening, the gap had shut. Inside the pocket sat the 4th and 9th Divisions, somewhere between nine and ten thousand men, with food and water already running short and nowhere left to go. A modern army had been folded shut like a hand closing around a fly.

The encircled army held for days, but a surrounded force in the waterless Chaco does not have days to spare. On 11 December the trapped divisions capitulated. The numbers are grim and the sources vary slightly, but the scale is not in doubt: up to two thousand Bolivian soldiers were killed and some seven thousand taken prisoner, along with thousands of rifles, hundreds of machine guns, and the divisions' artillery. Barely nine hundred men broke out, led by Captain Germán Busch, the same officer whose rearguard had saved the Bolivians at Gondra five months earlier. These were not professional soldiers in the main but conscripts, many of them Andean highlanders sent to die in lowland heat their bodies had never known.

Campo Vía broke something in the Bolivian war effort. In the weeks that followed, Paraguayan troops cleared the Bolivian army out of the eastern Chaco entirely. The German general Hans Kundt, a veteran of the First World War who had commanded the Bolivian forces through their costly frontal assaults, was forced from the high command, his reputation buried in the scrub he could not take. On the Paraguayan side, the victory carried Estigarribia upward: President Eusebio Ayala traveled to the front to promote him to general. The war would grind on for another year and a half, but its momentum had shifted, and it would never shift back.

There is nothing dramatic to see at Campo Vía now, only the same flat thorn forest and the same hard light that the trapped men knew. That emptiness is, in a way, the point. Wars are usually remembered by their monuments, but the Chaco swallowed most of its dead without ceremony, and the pocket where thousands surrendered is just another patch of Presidente Hayes Department. The captured weapons were carted off, the prisoners marched into Paraguay, and the scrub closed over the trenches. To fly over it is to look down on a landscape that gives no hint of what it once held: an entire army with no water and no way out, learning that in this country the terrain itself was the deadliest weapon either side could field. The men who surrendered here were among the lucky ones. They lived.