In Marne, France. This battle was the last major German offensive in World War 1. It was clear that Germany had failed in their attempt into win the war, in fact they had lost ground. Some of the German commanders, including Crown Prince Wilhelm, believed the war was lost by this point.
Erich Ludendorff, effectively the German Chief-of-Staff (although Paul von Hindenburg was the ostensible commander), was convinced that they could still win the war by an attack in Flanders. He aimed to lure Allied forces from Belgium to the Marne in order to divert their attention from the north where they intended to mount their major attack. After the Germans had ultimately failed in their efforts to break through, Ferdinand Foch, the Allied Supreme Commander, launched 24 divisions of the French army alone, in addition to U.S., British and Italian troops and 350 tanks. This counter-offensive was a complete success. On the 20th of July the Germans retreated, and by the 3rd of August they were back where they had started at the Aisne-Vesle rivers.
The casualties were high, especially for Germany, who suffered 168,000 casualties. France suffered 95,000, Britain 13,000 and the U.S. 12,000. As a consequence of the disastrous result in the Marne, Ludendorff's planned Flanders offensive cancelled, and there were no more large-scale attempts to win the war.