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 Europe
Some other notable dates are the Battle of Adrianople in 378, the death of Theodosius I in 395 (the last time the Roman Empire was politically unified), the crossing of the Rhine in 406 by Germanic tribes after the withdrawal of the legions to defend Italy against Alaric I, the death of Stilicho in 408, followed by the disintegration of the western legions, the death of Justinian I, the last Roman Emperor who tried to reconquer the west, in 565, and the coming of Islam after 632. Many scholars maintain that rather than a "fall", the changes can more accurately be described as a complex transformation. Over time many theories have been proposed on why the Empire fell, or whether indeed it fell at all.