The Army That Fought Itself
In 1788, the Austrian army suffered a massive defeat without seeing a single enemy soldier. A drunken dispute over schnapps turned into a chaotic friendly fire battle that killed thousands of their own troops.
During the Austro-Turkish War, Austrian reconnaissance units began fighting over alcohol supplies, accidentally shouting that the Turks were attacking. Panic spread like wildfire through the multi-lingual ranks, leading soldiers to fire on shadows and each other in the dark. By dawn, the army had retreated in disarray, leaving the real enemy surprised to find a decimated camp. It remains one of the most absurd and costly military blunders in history.
Source: Battle of Karánsebes