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.
History of the Roman Empire
The Western Roman Empire began to disintegrate in the early 5th century as Germanic migrations and invasions overwhelmed the capacity of the Empire to assimilate the migrants and fight off the invaders. The Romans were successful in fighting off all invaders, most famously Attila the Hun, though the Empire had assimilated so many Germanic peoples of dubious loyalty to Rome that the Empire started to dismember itself. Most chronologies place the end of the Western Roman empire in 476, when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer. By placing himself under the rule of the Eastern Emperor, rather than naming himself Emperor as other Germanic chiefs had done, Odoacer ended the Western Empire by ending the line of Western Emperors. The eastern Empire exercised diminishing control over the west over the course of the next century. The empire in the East—known today as the Byzantine Empire, but referred to in its time as the "Roman Empire" or by various other names—ended in 1453 with the death of Constantine XI and the fall of Constantinople to the Ottoman Turks.