They were able to expand their empire through sheer force. At some points, they were burned down by Gauls and had their armies defeated, but they continued to expand their empire by not giving up.
In the pre-republic era, a great example occurred. The Romans signed a treaty with their Latin neighbors saying that they would be ae\s as long as Earth and Heaven endured. Soon after, they betrayed their treaty through their conquest of the Italian Peninsula. They were hesitant to ally with anybody at some points, and glad to do so at others. It really depends.
