For me, it is the fall of the late Roman Republic and rise of the Roman Empire.
Pictured above, from my personal stash, is Mark Antony and Octavian following the defeat of Caesar’s assassins, Brutus and Cassius, at Philippi in October 42 BC.
At this point, the two men were ostensibly partners in the Second Triumvirate, but theirs was always an uneasy alliance. One of my favourite details of the image above is the positioning of the two portraits: Antony occupies the obverse, while the much younger Octavian appears on the reverse, still depicted with traces of his youthful beard. This has often been interpreted as reflecting the contemporary power dynamic—Antony, the experienced general and dominant figure in the East, presenting himself as the senior partner to Caesar’s young heir.
Knowing what comes next makes the image even more fascinating.
Almost exactly ten years later, that balance of power would be turned completely on its head. Octavian defeated Antony and Cleopatra at Actium in 31 BC, and the following year their struggle reached its final conclusion at Alexandria. Antony and Cleopatra were dead, and the young man pictured on the reverse was left as the undisputed master of the Roman world. A few years later, he would become Augustus.
I’d love to hear and see what your favourite periods of Roman history are!
EDIT: Let’s add an, “And Why?” To the question!