Geography made Humaitá formidable. Here the Paraguay River doubles back on itself in a tight horseshoe bend, and on the low cliff above it the Paraguayans built a sequence of gun batteries that could pour converging fire onto any ship trying to pass. A great chain boom could be raised across the channel to halt enemy vessels and trap them beneath the guns. The fortress controlled river access to the capital, Asunción, 275 kilometers upstream, which made it the cork in the bottle of the entire country. Foreign observers gave it a nickname that stuck: the Gibraltar of South America. Begun under President Carlos Antonio López, it became the linchpin of Paraguay's defense.

The Paraguayan War of 1864 to 1870 set landlocked Paraguay against the combined might of Brazil, Argentina, and Uruguay - the Triple Alliance. It became the deadliest conflict in the history of South America, and for Paraguay it was nearly the end. Scholarly estimates are staggering: a prewar population of roughly half a million was reduced to around 221,000 by 1871, with the loss of well over half the people. Among the survivors, women vastly outnumbered men, so many of the country's males had died in combat, of disease, of hunger, and of exposure. This was not a war Paraguay could win; it was a war it barely survived as a nation.

Humaitá frustrated the allied advance for more than two years, and the fighting around it produced some of the war's most terrible days. At Curupayty, just downriver, dug-in Paraguayan defenders shattered an allied assault in September 1866, inflicting thousands of casualties at almost no cost to themselves - a victory so lopsided it stalled the invasion for nearly a year. But the alliance had numbers and time, and slowly the fortress was encircled. When Humaitá finally fell in 1868, it was not stormed so much as strangled, its starving garrison forced out. Up to 24,000 troops had once sheltered behind these works; in the end, the town was left wrecked by the very war it had been built to stop.

Humaitá's name comes from the Guaraní words yma and itá - "ancient stone" - and the description fits a place that now lives mostly in memory. Portions of the old earthworks still scar the ground, and the gutted church draws students from across Paraguay and visitors from far beyond, come to stand among the vestiges of the war. The Cuartel de López, a museum in the former barracks of the Paraguayan commander Francisco Solano López, keeps the small relics of the fighting: bullets, cannon, stirrups, spurs, swords. The rest of the town has returned to a gentler rhythm. Fishermen pull surubí and dorado from the river, and life goes on quietly above the bones of the Gibraltar that once was.