Battle of Yrendagüé
1934 CE.
Shown on the map at -20.520°, -62.203°.
The Battle of Yrendagüé was fought amidst the Chaco War between Bolivia and Paraguay, in early December 1934. A Paraguayan division marched through the gap in the lines between two Bolivian divisions and captured the Bolivian Cavalry Corps' main water source, Fort Yrendagüé, 70 km away from the front. This left the whole Cavalry Corps, 12,000 men, without any viable water sources, and ultimately that unit disintegrated, most of its men being either captured by Paraguayan forces or dying of thirst in the Chaco.
Text from the Wikipedia article Battle of Yrendagüé, licensed CC BY-SA 4.0.