The second wife of Octavian (Caesar Augustus), Livia Drusilla, had two children, Tiberius and Drusus. Drusus married Antonia, the daughter of Mark Antony and Octavia, the sister of Octavian. They had two sons, Germanicus and Claudius. Drusus was a very successful general campaigning in what became Germany. However while on campaign he was thrown from his horse and died of his injuries a few days later. Caligula was a son of Germanicus.

• 12 CE: Gaius Caesar Germanicus was born in Antium, Latium, a town 56 kilometers from Rome. His mother was Agrippina, known historically as Agrippina the Elder. He had two older brothers.

  Caligula was a nickname given to him as a child by the soldiers commanded by his father, Germanicus, who took him on his compaigns and dressed him as a little soldier. It meant little soldier's boots.

• 19 CE: Caligula's father, Germanicus, died. Caligula is adopted by his uncle Tiberius.
• 23 CE: Caligula's older brother, Drusus Caesar, died.
• 31 CE: Caligula's older brother, Julius Caesar Nero, died.
• 33 CE: Caligula's mother, Agrippina the Elder, died.
• 37 CE: Caligula's uncle, Tiberius, who was effectively emperor but not in name, dies. The Praetorian Guard chose Caligula to succeed Tiberius. This was in preference to Tiberius' grandson, Tiberius Gemellus, who was thought to be Tiberius' heir to power.

  Caligula is about 25 years of age at his accession to power. He is severely ill the first seven months of his reign.

• 38 CE: Caligula has executed the head of the Praetorian Guard. Also the possible rival for the allegiance of the Preatorian Guard, Tiberius Gemellus is executed.

  Caligula's sister, Drusilla, dies. He has her declared divine as Diva Drusilla. Previously only men had been declared gods.

• 39 CE: Caligula made a surprise visit to the Roman army command on the Upper Rhine River where there was the possibility of a revolt of the army against him. He has executed the commander of the army there. He also has executed the husband of his dead sister, Drusilla.

  Caligula begins capricious and extravagant projects. Initially there were funds for them in the treasury that had been accumulated by his uncle Tiberius. When those funds were used up he began to extort funds from wealthy Roman citizens.

• 40 CE: Caligula takes an army into Gaul to confiscate wealth from the people there. He takes them north to edge of the English Channel supposedly to carry out an invasion of Britain but changes his mind and returns to Rome.
• 41 CE: In Rome the leaders of the Praetorian Guard kill Caligula and his wife and daughter. In their invasion of the palace the Praetorian Guard find Claudius, Caligula's uncle, hiding and trembling with fear. They decide that Claudius is a member of the royal family that they can control and they declare him to be the new emperor.