Roman emperor Caligula was murdered by Cassius Chaerea and other members of the Praetorian Guard, who proclaimed Claudius to be the new emperor. 40s: The 40s ran from January 1, AD 40, to December 31, AD 49. Following the assassination of Caligula, Claudius became Roman emperor in AD 41. In AD 43 he sent Aulus Plautius with four legions across the Channel, initiating the decades-long Roman conquest of Britain. January 24, 41. Wiki link