How did the Romans manage to effectively combine their whole empire into one unit?

I believe the Romans managed to effectively combine their empire into a whole unit because of their wars, the Pax Romana, and their roads and seaways. The Romans had many battles such as the battle with the Etruscans and the Carthaginians. After winning these battles the Romans captured their land, and used it for themselves. Nobody later captured their area back. After all these battles, and all these cities captured the Romans entered a time of peace called the Pax Romana. During this time the Romans made their roads and seaways which connected their city together making it easier for people to travel. Also during the Pax Romana, Romans got their crops in their city so they could stock up on money, and they got their money faster with the roads and seaways. After entering the time period of the Pax Romana, and stocking up on crops and money, i believe thats how the Romans effectively combined their city into one unit.