In a nutshell. The ground was boggy and wet. The battle start was delayed several hours because of the wet. Grouchy, although urged by his senior staff, refused to ride towards the sound of the guns, instead he chased Blucher. Prince Jeromes' diversionary attack (Hougomont) developed a momentum all of it's own, sucking in the left flank of the Grande Armee. At a crucial juncture Napolean became ill and left the battlefield. In his absence Ney counter attacked the British line with his cavalry. Just think horses v pikes you'll get the picture. Wellington kept his men on the reverse slope of the ridge, negating any artillary fire that wasn't screwed up because of the mud. The Prussians turned up about 4 in the afternoon. The Grande Armee ran away. Interesting trivia, Napoleans army still outnumbered the Allies even as they routed.

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