What feature of the Roman Republic was key in helping them build a stronger economy and army than the Greek polis?
William Harris of Columbia University has cogently argued that the rise of Rome was also a rise of imperialism. He comes to this conclusion in a few ways. First, he argues that the Roman Republic was structured in such a way as to create for healthy competition for glory among the elite. One of the ways in which the elite would get this glory was through victory in warfare. This point suggests that the very structure of the Roman constitution was designed for conquest. This is very different than the Greek cities. Second, the Romans were also able to assimilate conquered people into their body politic. In this way, Rome constantly grew. Again, the Greek did not do this as a practice. In fact, Greece almost ever assimilated people.
In terms of economic gain. Rome became wealthy especially through warfare. Whenever Rome won a military battle, they stole all of the wealth of the enemy, which was enormous. They also took the people and used them as slaves, which also made them very profitable. Finally, at times they even took their land. If you think about it, Roman military victory gave them free land and laborers. This was a perfect formula for wealth.