A Fly on the Wall

By the 1800s, the major powers had mostly decided that you couldn't just go about invading whoever you wanted on a whim - instead, you needed a justification (known as a 'casus belli') for an attack.  The problem with this was that everyone still wanted to go about carving up the world and painting the maps their color of choice.

In the 1820s, France looked across the Mediterranean, saw the ports of Algeria and said "man, I gotta get me some of that!"  The trouble was, they didn't have any particularly good reason why Algeria should be theirs - at least not besides the fact that France was big and who was going to stop them?  Luckily, there was a standing dispute between the nations that could be exploited - back in the 1790s France had contracted to purchase grain from some powerful merchants in Algeria, but had never paid.  The merchants, in turn, owed money to the ruler (called the Dey) of Algeria, and claimed they couldn't pay up until France paid them.

The argument over who should pay whom went back and forth for a few years before coming to a head at a meeting between the French ambassador and the Dey, in which the Dey became upset with ambassador's lack of answers, and smacked him with his fan/fly swatter.  As any schoolkid will tell you, "he hit me first!" is an excellent casus belli, and the French were not about to waste the opportunity.

After first blockading the Algeria ports for three years, they launched a full invasion in 1830 and began the occupation of the country.  It took decades of bloody war for French forces to establish control, and decades more to extend it deep into the desert territories.  And the French didn't half-ass the situation either: they went full-on brutal, oppressive colonial overlords.  One of the French commanders, Lieutenant-Colonel de Montagnac, wrote in 1843:

"All populations who do not accept our conditions must be despoiled. Everything must be seized, devastated, without age or sex distinction: grass must not grow any more where the French army has set foot. Who wants the end wants the means, whatever may say our philanthropists. I personally warn all good soldiers whom I have the honour to lead that if they happen to bring me a living Arab, they will receive a beating with the flat of the saber.... This is how, my dear friend, we must make war against Arabs: kill all men over the age of fifteen, take all their women and children, load them onto naval vessels, send them to the Marquesas Islands or elsewhere. In one word, annihilate all who will not crawl beneath our feet like dogs."

All this because someone got slapped with a fly swatter.