Emperor
The first Austrian Emperor was the last Holy Roman Emperor Francis II. In the face of aggressions by Napoleon, Francis feared for the future of the Holy Roman Empire. He wished to maintain his and his family's Imperial status in the event that the Holy Roman Empire should be dissolved, as it indeed was in 1806 when an Austrian-led army suffered a humiliating defeat at the Battle of Austerlitz. After which, the victorious Napoleon proceeded to dismantle the old "Reich" by severing a good portion from the empire and turning it into a separate Confederation of the Rhine. With the size of his imperial realm significantly reduced, Francis II, "Holy Roman Emperor" became Francis I, "Emperor of Austria". The new imperial title may have sounded less prestigious than the old one, but Francis' dynasty continued to rule from Austria and a Habsburg monarch was still an emperor ("Kaiser"), and not just merely a king ("König"), in name.
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.