He expanded his empire by offering citizenship to those he conquered.

When the Romans expanded into Italy, they made alliances with a number of Italic peoples, who supplied soldiers who supported Roman legions. This considerably increased the pool of military manpower available to Rome. Rome granted citizenship to leaders of the towns of the allied peoples who showed loyalty as a reward for for this. Following a war which was caused by a rebellion of the allies because Rome refused to grant them Roman citizenship, the Romans awarded such citizenship to them. Julius Caesar also granted Romans citizenship to the peoples of northern Italy which at the time was not considered to be part of Italy.

When Rome expanded beyond Italy she continued to grant citizenship to elite men in the conquered areas who showed loyalty to Rome.

In 212 AD the emperor Caracalla extended Roman citizenship to all freeborn men in the empire. Tacitus, an ancient Roman historian thought that he did this so that he could tax them as well.

Explanation:

protected civil liberties and outlawed slavery in the new territories.