In 1815, the Hundred Days campaign involved which European leader?
And the answer: Napoleon.
If the Hundred Days were a movie, it would be known as "Napoleon, Part Two." After more than a decade as emperor of France, Napoleon Bonaparte was forced to abdicate the throne and go into exile. However, less than a year later, he reemerged with an army and took over the empire for roughly 100 days, but was defeated for the second time at the Battle of Waterloo.
Although he was sent to exile in Elba, Napoleon remained in the loop of French affairs. By the time he had heard of the shrinking French empire and reinstalled nobility in Paris, Napoleon was ready to make one final attempt at the throne. Luckily, as a man who based his career on the ability to win over others, Napoleon convinced the French military to rally around him on the mainland by greeting them, unarmed, with the phrase: "Go ahead, shoot your emperor."
Louis the 18th soon caught wind of Napoleon's return, and escaped from Paris. By March of 1815, Napoleon effectively regained control of Paris and, more widely, France. This marks the beginning of the Hundred Days.
Needless to say, major European powers weren't too happy to see Napoleon. As such, they began to reorganize their military forces in an effort to force Napoleon out for good. The French king decided taking the offensive would be best, and launched a campaign against the combined powers of Great Britain and Prussia in what is now Belgium.
However, it all came down to the Battle of Waterloo. After an extremely bloody battle (where 40-50,000 people were killed or injured), Napoleon lost. Some believe that the ground conditions of the battle contributed to Napoleon's loss, as it favored defensive positions over that of the offensive. After the battle, the French retreated to Paris, where Napoleon surrendered.
Learn more about Napoleon's Hundred Days below.