How did Constantine I reorganize the Roman Empire’s civil administration?

Constantine I significantly reorganized the Roman Empire’s civil administration by building upon the foundations laid by his predecessor, Diocletian, while introducing structural changes that defined the later imperial period.12

Key aspects of his administrative reorganization included:

• Separation of Civil and Military Authority: Constantine formalized the division between civil and military careers. He replaced the traditional praetorian prefect—who had previously held both civil and military power—with regional praetorian prefects who exercised civil and financial authority exclusively. Military command was transferred to new magistri (masters) of the infantry and cavalry.123
• Central Administration: He expanded the civil service significantly, creating a new nobility of imperial service. Key roles in the central administration were granted to the palace quaestor, the magister officiorum, and the counts of finance (comes sacrarum largitionum and comes rei privatae).1
• Regional Prefectures: He established vast regional prefectures—specifically Gaul, Italy, Illyricum, and the East—to which diocesan vicars were made responsible.1
• Capital Relocation: By establishing Constantinople as his new imperial capital in 330, he created a second administrative center, complete with its own senate and proconsul, to better manage the empire's strategic and economic needs.13
• Fiscal Reforms: To support the state, he introduced new taxes, such as the collatio lustralis on merchants and craftsmen, and minted the gold solidus, which became the stable foundation of the Byzantine Empire’s currency.132