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

The Romans managed to effectively combine their whole empire into one unit by doing one thing; having a common goal. For the Romans, their common goal was to keep their land and become a greater empire. This helps unite the empire because everyone comes together when they believe in something or want something badly. Just like when Hitler united the Germans, even though their were some civil arguments amongst themselves, by using his capturing speeches he lured all the Germans to unite and kill millions of Jews. As does the Romans, with whomever threatens them and their land.