January 24, 41 CE. Shortly after declaring that he was setting himself up as a living god, a group of Senators and Praetorian Guards violently stab Roman Emperor Caligula to death. He is the first sitting Emperor to die by assassination, but my GOODNESS he is not the last.
Caligula is a nickname: it means "little boots," based on the fondness the legionaries had for him as a young boy. His true name was Gaius Julius Caesar, which is, yes, confusing. The Romans recycled names a lot. He was actually the grandson of Augustus, the famous Julius Caesar's adopted son. This is confusing, but this is Rome - they like it that way. Historians call him Caligula because when you start having to specify "Caesar, but not those other twenty Caesars" things get downright esoteric.
Caligula was the third Roman Emperor, successor to his adoptive grandfather Tiberius, and this is where we get to the point that insanity had begun to pop up in the Julio-Claudian family. Augustus was a stone-cold genius, brilliant to the end. Tiberius started out well, but descended into paranoia and scheming in his last several years. Caligula only waited six months before turning the crazy up.
Tiberius' last several years had seen the murder of several dynasty members, and when Caligula succeeded to the throne the citizens of Rome breathed a sigh of relief. The reign of terror was over! Six months into his reign, however, Caligula fell ill. He survived, but came out of the illness...different. Worse. He immediately started executing other family members. This would develop into one of the favorite pastimes of the Roman Emperors. The only one he spared was his "simple" Uncle Claudius, who he thought a laughingstock.
Then it got worse.
Caligula was described by his contemporaries as insane. Popular amongst the people, who just saw the building projects and improvements he built, the Senatorial class and the military despised him. The Roman people had adored him since he was a boy in the Imperial family; the affectionate nickname "Caligula" was still what they called him.
Reports come down to us of Caligula's forced incest with his sisters, casual murder, paranoid jealousy, and gleeful tyranny. At one point he is alleged to have casually raped a senator's wife in front of him at a dinner party. On another occasion, he had a section of the Colosseum's crowd thrown to the lions because it made a better show. During an attempted invasion of Britain, the sea was too rough to cross the Channel; he had his soldiers stab the waves to punish the ocean for its impertinence, and gather shells to "harvest the bounty of the sea."
Most famously, he tried to make his horse Consul. We know that his horse did become a priest.
Finally, and most offensively, he threatened to declare himself a living God.
Or...was all that true?
It is, of course, possible that Caligula's popularity with the people worried the upper classes. It would be interesting if they spread stories of sexual improprieties as justification for their future actions...an extremely common practice in the ancient world. It would be funny if they exaggerated his insanity and blew questionable decisions out of proportion. The story about stabbing the sea, after all, is suspiciously similar to a story from the Greek historian Herodotus of Emperor Xerxes having his men whip the ocean for its disobedience. It's even worth asking whether the horse thing wasn't a pointed joke (As in, "My horse could make a better Consul than you, Flaccus.")
Either way, on January 24, 41 CE, some senators and Praetorian Guards went ham on poor crazy Caligula. After they tied up some other loose ends - loose ends being the gruesome murder of Caligula's wife and young daughter, who had her head smashed against a wall - they found poor Uncle Claudius hiding in a closet. The fool that Caligula had spared because he was no harm to anyone was now Emperor Claudius.
To everyone's surprise, Emperor Claudius was pretty decent. Lot less crazy.
Also, he was assassinated too. That just became the norm from now on.
That, and many other reasons, is why we don't do monarchy anymore.
A popular account of this period in Roman history is Tom Holland's Dynasty: The Rise and Fall of the House of Caesar (New York: Random House, 2015). Fair warning that Tom Holland (not the actor) is a very glammy pop historian, fairly well-known for fudging a fact here and there. Michael Grant, on the other hand, was a famous and experienced classicist known best for his writing and for his refusal to talk down to his audience. Try The Twelve Caesars (New York: Penguin Books, 1975).