The article offers a general overview of the political situation in the Roman Empire during the third century, the main prerequisites for the emergence of the crisis of power that affected many spheres of state and public life. The relevance of this work can be explained by the possibility of understanding past events as previous experience, the ability to predict the processes of transformation of the state and society and the use of such examples for the most favorable choice of action in the future. The author studies a series of events that is designated as the crisis of the 3rd century. The Roman Empire, the period of the rule of many emperors, succeeding each other from year to year, military reforms, decentralization of power and internal struggle for power, determine the main features of the transition from the principate to the dominant. Transformations under consideration are divided into military reforms that require changes due to failures in the border territories of the empire, as well as the inability to stop the hordes of barbarians constantly invading the lands of the empire, and administrative reforms in the system of government, decentralization of power, and foundation of a new political institution of government — a tetrarchy. The author concludes about the significance and scale of transformations carried out at the beginning of the 3rd century for the history of the state and law, emphasizing the pace of those reforms due to domestic and foreign policy circumstances.