The situation
Back from exile, Napoleon struck into Belgium to defeat the British-led and Prussian armies separately before they could combine. On 16 June he beat the Prussians at Ligny and sent Marshal Grouchy with about 33,000 men to pursue them.
The decision
On the morning of 18 June the ground was soaked from overnight rain. Napoleon delayed his main attack until late morning to let it dry enough for artillery and cavalry. The wait mattered: every hour brought the Prussians closer.
Wellington held a ridge, with the farms of Hougoumont and La Haye Sainte as strongpoints. Massed French infantry and cavalry attacks went in through the afternoon. By early evening Prussian columns were striking the French right at Plancenoit, and the final assault of the Imperial Guard was thrown back.
What it cost
Combined casualties were roughly 50,000 killed and wounded in a single day. Napoleon abdicated and was exiled to Saint Helena, where he died in 1821.