Question: What factor is considered by some to be the main driving force behind the expansion of Rome?
Sentence: As such, early republican Rome was not an "empire" or "state" in the modern sense, but an alliance of independent city-states (similar to the Greek hegemonies of the same period) with varying degrees of genuine independence (which itself changed over time) engaged in an alliance of mutual self-protection, but led by Rome.
Answer in sentence: no