Early modern period
In the early modern period, the Holy Roman Empire was a union of territories in Central Europe under a Holy Roman Emperor the first of which was Otto I. The last was Francis II, who abdicated and dissolved the Empire in 1806 during the Napoleonic Wars. Despite its name, for much of its history the Empire did not include Rome within its borders.
East Roman army
In 395, the death of the last sole Roman emperor, Theodosius I (r. 379-95), led to the final split of the empire into two political entities, the West ("Occidentale") and the East ("Orientale"). The system of dual emperors (called "Augusti" after the founder of the empire, Augustus) had been instituted a century earlier by the great reforming emperor Diocletian (r.284-305). But it had never been envisaged as a political separation, purely as an administrative and military convenience. Decrees issued by either emperor were valid in both halves and the successor of each "Augustus" required the recognition of the other. The empire was reunited under one emperor under Constantine I, after 324, under Constantius II, after 353, under Julian, after 361, and Theodosius himself, after 394.