Complex sentence: The battle was the last Bolivian attempt to capture the heavily fortified "fortín" of Nanawa, a major defensive complex, by capturing Nanawa Bolivians hoped to breach to Isla Poí, the Paraguayan headquarters and ultimately reach Paraguay River.
Split into simpler sentences: The battle was the last Bolivian attempt to capture the heavily fortified "fortín" of Nanawa, a major defensive complex. By capturing Nanawa Bolivians hoped to breach to Isla Poí, the Paraguayan headquarters, and ultimately reach Paraguay River.