On this day, 20th of March 1815, Napoleon Bonaparte returned from his island exile and waltzed into Paris unopposed.
He didn't stick around for long though, lasting only 100 Days (Cents Jours) before being booted away to another, different island. Napoleon's rise was quite meteoric, so it only makes sense for his fall to be similar.
Despite being pitted against all his neighbours, Napoleon managed to rebuff his monarchist enemies and even capture land for his Republic. Obviously the sovereigns couldn't have this upstart disrupting the regime! A Coalition of Six Countries finally defeated Napoleon at the Battle of Montmartre and deposed him. He was forced into retirement on the Island of Elba.
Now obviously Napoleon wasn't happy with that, who would be? He lasted just under 10 months on Elba, slipping away on a ship while the Coalition squabbled over the division of land from the war.
He landed on the mainland with some 1000 soldiers, and began to make his way (avoiding royalist Provence) to Paris. French defensive armies were sent to intercept, but ended up only joining his ranks. Napoleon's forced swelled to an army.
From there, he basically just walked into Paris with a warm welcome ceremony. Unbeknownst to Napoleon, this would begin his last 100 days in France.
Clearly this did not sit so well with the Coalition. Another war started almost immediately, and it lasted (as you might have guessed) 100 days. The campaign began and ended with the famous Battle of Waterloo, but that's an article for another day.
Napoleon was shipped of to the Island of Saint Helena, the Monarchy was restored, and order returned to France.
For a little while, at least.