How and when did the transition from republic to empire take place in Rome?

How and when did the transition from republic to empire take place in Rome? What stages in the development of the empire do historians identify?

The immediate transition to the empire took place with the establishment of the power of Octavian Augustus in Rome, who gradually took over all the threads of governing the Roman Republic. Under his successors, the imperial power is strengthened.

The following stages are distinguished:

1. The period of the principate: from the reign of Octavian Augustus to the assassination of the last emperor of the Sever dynasty (Alexander Severus);

2. Crisis of the III century: numerous emperors appear, who are chosen by the army, civil wars and raids by barbarians destroy the empire;

3. Dominate period: began during the reign of Diocletian. Reorganization of the empire.