(Note: The next paragraph was copied from Wikipedia, with small changes)
In the Battle of Waterloo (Sunday, 18 June, 1815, near Waterloo) forces of the French Empire under Napoleon Bonaparte and Michel Ney were defeated by those of the Seventh Coalition, including a Prussian army under the command of Gebhard von Blücher and an Anglo-Allied army under the command of the Duke of Wellington. It was the decisive battle of the Waterloo Campaign and Bonaparte's last. The defeat at Waterloo put an end to Napoleon's rule as the French emperor, and marked the end of Napoleon's Hundred Days of return from exile.
This battle played out almost identically to the OTL battle.
Napoleon's defeat allowed the British to concentrate all of their forces on the war in North America, where the Americans were trying to regain New Orleans and also take control of the British Empire's colonies.