Rome went from being one of many city-states in the Italian Peninsula to being the center of the most powerful empire in the world between the fifth century BCE and the first century CE. During the Republican period of Rome, political offices and institutions were designed to prevent any one man from becoming too powerful. These systems began to break down in the first century BCE.
Rome was able to gain its empire in large part by extending some form of citizenship to many of the people it conquered.
Military expansion drove economic development, bringing enslaved people and loot back to Rome, which in turn transformed the city of Rome and Roman culture.