Around 400 BC as a practical matter, in that it was becoming obvious by that stage that Rome was expansionist and might in due course occupy quite a bit of Europe and the Mediterranean coasts.
But it depends what you mean by "a thing". You could ask the same fairly vague question about all of the empires etc which have existed in the world. No single date, nor even a single century, could be quoted without some historians objecting.
it coincided with the decline of Greece which became all chiefs and no indians some of these Chiefs had to have been at the heart of the rise of Rome to the world stage the cultures were practically identical 400 BC