This battle was fought in Waterloo, Belgium on June 18, 1815. This is an important battle as it is the battle in which Napolean was defeated, thus ending his expansion plans for his empire. In this battle, Napolean fought against an army with soldiers from all over Europe, including but not limited to, the UK, Prussia, and the Netherlands. The reason he lost was due to a critical strategy error on Napolean's part. He waited until midday to order his troups to attack the enemy, after waiting for the waterlogged battleground to dry up more. Because of this, the forces Napeolean was fighting had time to get fortification with nearly 30000 more soldiers coming to aide in the fight against the French. It was because of this that Napeolean did not have enough forces to survive fighting against his enemies, and by the end of the day he had lost. This battle signified the end of his 100 day long march in which he expanded his control and power, taking over areas surrounding France in order to grow his empire. 

Source: https://www.history.com/topics/british-history/battle-of-waterloo