The exact causes and motivations for Rome's military conflicts and expansions during the republic are subject to wide debate. While they can be seen as motivated by outright aggression and imperialism, historians typically take a much more nuanced view. They argue that Rome's expansion was driven by short-term defensive and inter-state factors (that is, relations with city-states and kingdoms outside Rome's hegemony), and the new contingencies that these decisions created. In its early history, as Rome successfully defended itself against foreign threats in central and then northern Italy, neighboring city-states sought the protection a Roman alliance would bring. 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. With some important exceptions, successful wars in early republican Rome generally led not to annexation or military occupation, but to the restoration of the way things were. But the defeated city would be weakened (sometimes with outright land concessions) and thus less able to resist Romanizing influences, such as Roman settlers seeking land or trade with the growing Roman confederacy. It was also less able to defend itself against its non-Roman enemies, which made attack by these enemies more likely. It was, therefore, more likely to seek an alliance of protection with Rome.

What factor is considered by some to be the main driving force behind the expansion of Rome?