Augustus becomes Rome's first emperor
Octavian, Julius Caesar's adopted heir, won Rome's civil wars and was given the title Augustus by the Senate. Under him the Mediterranean world entered two centuries of relative peace, the world into which Jesus was born.
But when the fullness of the time came, God sent out his Son, born to a woman, born under the law
Galatians 4:4 (World English Bible)
After defeating Antony and Cleopatra at Actium in 31 BC, Octavian was master of the Roman world. In January 27 BC the Senate honored him with the name Augustus, “revered one.” He reigned another forty years, and it was his decree that all the world should be enrolled that brought Joseph and Mary to Bethlehem (Luke 2:1).
Paul wrote that God sent his Son when the time had fully come (Galatians 4:4). Looking back over the four centuries since Malachi, Christians have seen God preparing the way: a common language spread by Alexander, the Scriptures translated into it, synagogues in cities across the empire, and Roman roads and peace that let the apostles travel freely.
Dating notes
Ussher: January 27 BC, the conventional date as well. Luke 2:1's decree is discussed with Jesus' birth in the next era.