Even then, Rome was expansionist in terms of territory it directly controlled. All viable comparisons between the Roman Republic and America ended after we hit the Pacific. Rome hit the Mediterranean and kept on going.
It's not the conquest of territory that made the republic turn into an empire.
The dissolution of the republic started when people realized that they could just blow off the constitution and get whatever they wanted by either paying off the population or just using sheer force. We have seen echoes of this in our recent elections. Once 48% of the population becomes convinced that elections aren't important to have unless they get to win, and they would rather have a tyrant that's on their side than an elected government, well, let's just say I'm not optimistic about the outcome.