(Note: The next paragraph was taken from Wikipedia, with small changes)
The Hundred Days, sometimes known as the Hundred Days of Napoleon or Napoleon's Hundred Days for specificity, marked the period between Napoleon Bonaparte's return from exile on Elba to Paris on 20 March 1815 and the second restoration of King Louis XVIII on 8 July 1815. This period is also known as the War of the Seventh Coalition, and includes the Waterloo Campaign and the Neapolitan War.
After Napoleon's defeat, the major European powers all rested and rebuilt except for Great Britain, which was dealing with the War of 1812 in North America. During the Hundred Days, the United States of America had redeclared war on the British Empire and taken advantage of Britain's distraction. Britain sent the majority of its soldiers to the war and ended the war.