He was declared emperor by the Praetorian Guard after Caligula's assassination

It is a fact that he was declared emperor by the Praetorian Guard after Caligula's assassination. Claudius' infirmity likely protected him from the fate of many other nobles during Tiberius and Caligula's purges, as potential enemies did not perceive him as a major danger. Following Caligula's assassination, he was declared emperor by the Praetorian Guard, making him the only adult male in his family.

Caligula was slain on January 24, 41, in a broad-based conspiracy involving Cassius Chaerea, a military tribune in the Praetorian Guard, and numerous senators. There is no evidence that Claudius had a direct role in the assassination, though it has been suggested that he was aware of the scheme, especially because he fled the crime scene just before his nephew was murdered. However, following the deaths of Caligula's wife and daughter, it became clear that Cassius planned to go beyond the conditions of the conspiracy and completely destroy the Imperial dynasty.

Claudius observed the German guard slaughter several uninvolved noblemen, including several of his friends, in the mayhem that followed the murder. He ran away to the palace to hide. According to legend, a Praetorian named Gratus discovered him hiding behind a curtain and unexpectedly pronounced him the prince. The Senate promptly convened and began debating a change of administration, but this quickly turned into a debate about who would be the new prince. When they learned of the Praetorians' allegation, they asked that Claudius be brought before them for approval, but he refused, fearing the consequences of complying. According to certain historians, particularly Josephus, Claudius' activities were directed by the Judaean King Herod Agrippa. However, an earlier version of events by the same ancient author minimizes Agrippa's role, leaving the situation ambiguous. The Senate was eventually compelled to cave. In exchange, Claudius offered broad amnesty, notwithstanding the fact that he executed a few junior officers participating in the conspiracy. The actual assassins, including Cassius Chaerea and Julius Lupus, the murderer of Caligula's wife and daughter, were executed to assure Claudius' safety and to serve as a warning in the future.