Antony became ally's with Cleopatra. At the battle of Actium Antony's army and navy were defeated by Octavian's and at age 32 he was named emperor of the roman world. The republic had come to an end and in return the roman empire was formed. In the roman empire the military was growing weak, the economy was falling and a lack of loyalty fell amongst the empire. Diocletian and Constantine passed reforms to help fix Rome like fixing the prices of goods and doubling the size of the military etc... Diocletian had realized that Rome was too large and divided it into two separate parts. The authoritarian policies we temporarily successful but it all came to an end when Constantine reunited the roman empire. After Constantine's death the empire redivided. Fighting occurred continually between the states. The west collapsed and became Germanic kingdoms. Soon after Christianity was introduced, a religion in which beliefs were opposite the traditional roman religion which was polytheistic. Christian belief in one god -who was not the emperor- undermined the authority and integrity of the emperor. The declines and collapses of the roman republic and the roman empire are similar because they both involved weakening in central government.
Ergo there may be differentiations between the political structures of the roman republic and the roman empire due to various discrepancies within central government but social