The expansion of the Roman Empire, from the end of the Germanic Wars of Augustus to the Consolidation of Hadrian, can be divided into five parts:
- the annexation of a series of client kingdoms early on (Numidia, Maurentina, etc)
- the Conquest of Britain
- the reorganization of the East following the Armenian War and First Jewish Revolt
- the subjugation and subsequent conquest of Dacia
- Trajan’s Eastern Campaigns (his war with Parthia and conquest of Arabia)
It strikes me that some of these can be easily averted. Now I’m not sure about this being its own thread -- narrower questions like “What if Rome never conquered Britain?” make for good enough topics, but I’m not sure if generalizing the conversation would work.