13. Battle of Tannenberg (1914)
In August 1914, during World War I, the Russian Second Army, commanded by General Aleksandr Samsonov, advanced into East Prussia without proper coordination with the First Army and secure communication. This lack of coordination and intelligence failures led to the German Eighth Army, under Generals Paul von Hindenburg and Erich Ludendorff, encircling and nearly destroying the Russian forces. The defeat resulted in approximately 70,000 to 120,000 Russian casualties and significantly impacted Russian morale early in the war. (britannica.com)
