Towards the end of Caligula's reign, he seemingly set his sites on military glory. Having never been involved in military achievement of any kind, such a step was another grandiose way to show his godliness to the people of Rome. Following in the steps of his father, Germanicus, Caligula launched a strange campaign into Germania. However, this campaign seems to have been interrupted by a conspiracy against him.
Though the entire affair is shrouded in uncertainty, a Legate of the Germania Legions, Cn. Lentulus Gaetulicus and Caligula's brother-in-law, M. Aemilius Lepidus, were executed. His surviving sisters, too, were exiled. As a result, the Germania expedition essentially didn't take place, and it only leaves us a few more claims of excessive behavior from the ancient sources. After nothing came of the Germania affair, Caligula apparently shifted his focus to Britain. Again, the results were nothing but laughable. According to Suetonius, rather than actually cross to Britain to achieve his goals of conquest he simply marched the legions to shore in some sort of show of strength.
"Finally, as if he intended to bring the war to an end, he drew up a line of battle on the shore of the Ocean, arranging his ballistas and other artillery; and when no one knew or could imagine what he was going to do, he suddenly bade them gather shells and fill their helmets and the folds of their gowns, calling them "spoils from the Ocean".
Despite this complete waste of time and resources, Caligula demanded a triumph from the Senate, which of course was awarded. Included in the complete mockery were Gauls dressed as Germans and the spoils taken from the shore. Some suggested it was indicative of Caligula's belief of personal divinity, that he actually defeated Neptune on the shores of the English Channel. According to Dio Cassius he even took the names Germanicus and Britannicus as if he had actually conquered those territories.
After his botched military campaigns, Caligula's megalomania apparently intensified dramatically, as well as his paranoia. Executions and torture were prevalent, and Rome's treasury was dangerously depleted. Despite his annexation of Mauretania as an official province (which is largely ignored in the ancient sources) Caligula's short reign had little benefit to the Roman people. His demand that a statue of his own likeness be erected in the temple of Jersusalem caused terrible riots in Judaea. Only the delaying tactics of the Governor Petronius and Herod Agrippa prevented wide-spread revolt.
Caligula's worsening behavior led to several plots against him, only one of which obviously needed to succeed. The plot that ultimately did succeed was developed, according to all ancient sources, within the Praetorian Guard. However, evidence also suggests that several Senators had knowledge and were involved in its undertaking. Apparently already under suspicion by Caligula, his Praetorian Prefects seemingly had little choice but to authorize the deed themselves or face certain execution. On January 24, AD 41, Cassius Chaerea, a Praetorian officer along with two others, brutally stabbed Caligula in a secluded corridor of the imperial palace. At the age of 28 years, and having ruled for just less than 4 years, Rome faced its first potential crisis of the Principate. The Praetorians continued their growth as a political institution since the days of Sejanus by directly executing or removing an emperor for the first of what would eventually be many times.
Though certainly some of Caligula's guard remained loyal, they were terribly outnumbered. The Praetorians swept the palace and butchered the Empress Caesonia and their baby daughter Drusilla had her head smashed against a wall. As the story goes, Claudius, the stammering old fool and uncle of the emperor, was found cowering behind some curtains. Seen as a choice to be easily controlled, the Praetorians swept him up and positioned him as the only surviving member of the Julio-Claudian line with a legitimate claim to the throne.
However, Claudius, by virtue of his survival through the terrible reign of his nephew and his later exemplary rule, seems more likely to have been a part of the plot seeking power of his own, rather than a frightened 'unlikely' heir, cowering behind some curtains. Regardless Caligula was dead and while the Senate perhaps sought a Republican return, Claudius was taken to the safety of the Praetorian camp. Whatever intentions the Senate may have had, without the loyalty of the legions and the Praetorians, their cause was doomed. They had no choice but to hail Claudius as the next 'Caesar'.