Conflicts that afflicted the last century of the Roman republic (88 bc–c.28 bc) and led to the inevitable institution of the unchallenged authority of one man, the Principate. Political life in Rome was unsettled from the period of Sulla’s dictatorship and the Catiline conspiracy (64–63 bc). Rivalry between the republican military leader Julius Caesar and Pompey began after the collapse of their alliance. Caesar defeated the Pompeian army in Spain at Ilerda (49 bc) and Pompey himself at Pharsalus (48 bc); he won further victories in Asia and Africa. Cato’s suicide in 46 bc signified the collapse of the republican cause. On his return to Rome, Caesar was made dictator and virtually sole ruler. His plans for safeguarding the empire by military expeditions against Dacia and Parthia were cut short by outraged republican traditionalists who murdered him in 44 bc. Further civil wars followed. Initially Octavian (Augustus), supported by the republican party, struggled against Mark Antony. In 43 bc Antony, Octavian, and Lepidus formed a coalition whose forces defeated the republicans led by Brutus and Cassius at Philippi. Antony meanwhile joined forces with Cleopatra and was defeated by Octavian at Actium. The Roman world was united under the sole leadership of Octavian, who annexed Egypt.

In 68 ad civil war broke out in the empire in the struggle for succession after Nero’s death. Galba was proclaimed emperor from Spain; he entered Rome in September but was murdered and succeeded by Otho; meanwhile Vitellius was proclaimed emperor in Germany and Otho committed suicide. Vespasian then invaded Italy and took the throne, making 68–69 “the year of the four emperors”. This crisis period was followed by the settled rule of Vespasian.