In her roads and fortifications, in the living rampart of
her legions, Rome long found security. Except for the districts conquered by
Trajan but abandoned by Hadrian, the empire during this period did not
lose a province. For more than two hundred years, throughout an area as large
as the United States, the civilized world rested under what an ancient writer
calls "the immense majesty of the Roman peace." 
 Pliny, Natural History, xxvii, 1.
EXTENSION OF ROMAN CITIZENSHIP
The grant of Roman citizenship to all Italians after the
Social War only increased for a time the contrast between Italy and the
provinces. But even before the fall of the republic Caesar's legislation had
begun the work of uniting the Roman and the provincial. More and more the
emperors followed in his footsteps. The extension of Roman citizenship was a
gradual process covering two centuries. It was left for the emperor Caracalla,
early in the third century, to take the final step. In 212 A.D. he issued an
edict which bestowed citizenship on all freeborn inhabitants of the empire.
This famous edict completed the work, begun so many centuries before, of
Romanizing the ancient world.