I don't know if this TL shouldn't be moved in the ASB forum...
Ok here we go:
1. A little Latin city can liberate without foreign help from a prosperous civilization, the Etruscans.
2. This little Latin city gets, by chance (?), a stable constitution, avoiding revolutions and civil wars.
3. The little republic is some centuries later besieged and sacked by Celts; however, Roma manages to conquer the whole Italian peninsula within around 100 years, using a unique combination of arms and tactics unknown to the ancient world.
4. The little republic, inexperienced in naval warfare, builds a superior fleet within some years and defeats the leading seafaring of the era. It also manges to defeat a military genius without loosing it's empire or even parts of it.
5. Rome, almost unchallenged, conquers everyhing.
6. Three civil wars, a big slave rebellion and a war with the main allies, beeing equal in military ability, do some damage, but the empire is meanwhile capable to conquer a huge territory (Gaul) in only 8 years and nobody mances the empire seriously during this crisis.
7. A man creates a republican monarchy and stabilizes the empire for 200 years.
8. Rome conquers more than everything.
So... divine intervention? ASB? Or a very very wanked TL?