Rome's military might unified the empire politically. A key factor in its succession of triumphs was a willingness to innovate. For example, in the First Punic War, Rome initially had no navy, while Carthage ruled the waves. However, during the course of the war, Rome learned how to build warships and how to use them effectively. Rome eventually defeated the Carthaginian navy. Innovative strategy also contributed to Rome's success. During the Second Punic War, a military leader called Fabius used guerrilla tactics to ward off defeat, and another military genius called Scipio used innovative strategy to defeat their enemy Hannibal in an important battle in northern Africa.
The Mediterranean Sea unified the Roman Empire, since most imperial territory lay along its shores. By sailing the Mediterranean, the different parts of the Roman Empire could keep in touch with each other, except during stormy weather. In addition, the Romans unified their empire by an extensive network of excellent roads.
At first, Rome hesitated to grant citizenship to conquered peoples. Its Italian allies did not receive Roman citizenship until they fought the Social War with Rome in the first century B.C. However, Rome eventually united its empire by the bond of a common citizenship. In 212 A.D., an emperor called Caracalla issued a decree granting citizenship to all free people in the empire.
Before the Roman conquest, Greek culture had already unified the eastern Mediterranean lands to some extent. The western portion of the empire adopted the Latin language, and such areas as Spain, France and the western part of northern Africa became thoroughly romanized. Further cultural unification took place as eastern and western sections of the empire mutually influenced one another. For example, the Romans made copies of Greek statues. The Roman poet Horace used Greek meters in his poetry, and Virgil modeled his "Aeneid" on the epics of Homer. In addition, the construction of such structures as basilicas, amphitheaters, public baths and aqueducts in various parts of the empire fostered cultural unity.
The peoples of the Roman Empire had many different religions, but syncretism prevailed in the ancient world. Syncretism is mixing different religions together and identifying various gods with each other. Rome tried to unify the religious scene by adding emperor worship to this religious brew. This strategy was successful in part but ultimately failed because of the opposition of non-syncretistic religions, especially Judaism and Christianity. After the emperor Constantine converted to Christianity in the fourth century A.D., Rome attempted to use the faith as a unifying force in the empire. While partially successful, these unification efforts faltered as a result of the development of variant forms of Christianity, such as Donatism, Arianism, Nestorianism and Monophysitism, which resisted imperial attempts at religious unification.