Black Day of the German Army: Battle of Amiens, WWI

On August 12, 1918, during WWI, the Allies defeated the Germans at the Battle of Amiens. It was the beginning of the 100 day offensive which signaled the defeat of the Germans the Western Front. An excerpt from the article:

"By August 11, the Germans had returned to their original, pre-Spring Offensives lines. Dubbed the "Blackest Day of the German Army" by Generalquartiermeister Erich Ludendorff, August 8 saw a return to mobile warfare as well as the first large surrenders of German troops. By the conclusion of the first phase on August 11, Allied losses numbered 22,200 killed wounded and missing. German losses were an astounding 74,000 killed, wounded, and captured. Seeking to continue the advance, Haig launched a second assault on August 21, with the goal of taking Bapaume. Pressing the enemy, the British broke through southeast of Arras on September 2, forcing the Germans to retreat to the Hindenburg Line. The British success at Amiens and Bapaume led Foch to plan the Meuse-Argonne Offensive which ended the war later that fall."