Just like a Hollywood movie that doesn't know when to end, Napoleon escaped from Elba, and returned for one last attempt at world domination. The memorial at the station refers to the "Allied armies" which rather recalls the WW2 term for the good guys. In 1815 these were: Austria, Prussia, Russia and the UK. Our picture source, the BBC, has a pretty good timeline for the Battle, which the Allies won, by the way.
Waterloo, once countryside in the Netherlands, is now a suburb of Brussels in Belgium.
For the story of how the news of the victory at Waterloo reached London see The Waterloo Way.
2022: The Guardian reported on the on-going mystery of what happened to the dead. Tens of thousands of men and horses died but the bones seem to have disappeared. It was thought that the bones were collected and pulverised into fertiliser for agricultural use. Academic archaeologists have been researching reports from the time and are planning a visit to the battlefield to see if they can find some graves.