History of the Roman Empire from this point on, leading his biographers to label him as insane. Most of what history remembers of Caligula comes from Suetonius, in his book "Lives of the Twelve Caesars". According to Suetonius, Caligula once planned to appoint his favourite horse Incitatus to the Roman Senate. He ordered his soldiers to invade Britain to fight the Sea God Neptune, but changed his mind at the last minute and had them pick sea shells on the northern end of France instead. It is believed he carried on incestuous relations with his three sisters: Julia Livilla, Drusilla and Agrippina the Younger. He
Roman emperor but only with Vespasian did "imperator" (emperor) become the official title by which the ruler of the Roman Empire was known. The word "princeps" (plural "principes"), meaning "first", was a republican term used to denote the leading citizen(s) of the state. It was a purely honorific title with no attached duties or powers. It was the title most preferred by Caesar Augustus as its use implies only primacy, as opposed to another of his titles, "imperator", which implies dominance. "Princeps", because of its republican connotation, was most commonly used to refer to the emperor in Latin (although the emperor's actual