Fortín Boquerón is the site of one of the decisive battles of the Chaco War (1932–35). There is an excellent museum, as well as a graveyard of the fallen and a gigantic monument constructed from the original defenses and trenches. The site is 65km south of a turnoff at Cruce Los Pioneros on the Ruta Trans-Chaco.
Look for the hollowed-out palo borracho tree once used as a sniper's nest. From the front it looks like a woodpecker hole but, despite being gutted more than 70 years ago, the tree is still alive!