Why would the romans mark the start with Julius when the senators killed him for it? They didn’t mark the start, Augustus self-declared, as Julius was doing. After Brutus and Cassius were defeated who would disagree with Augustus?
In my readings of Livy etc. The Romans took over various tribes, cities and territories and greatly expanded, as I referenced. They didn’t say hey look at the Roman Empire. They just called it Rome and referred to various provinces.
When we look back at the geography it is a simple mater to call it an empire whether the senate or the emperor ruled at a given time.
“Augustus self-declared,...”
Not really. He NEVER called himself “Emperor” iirc.
The historian Werner Eck states:
The sum of his power derived first of all from various powers of office delegated to him by the Senate and people, secondly from his immense private fortune, and thirdly from numerous patron-client relationships he established with individuals and groups throughout the Empire. All of them taken together formed the basis of his auctoritas, which he himself emphasized as the foundation of his political actions.