There are a lot of Roman successor states.

This probably doesn't get all of them, but I made an effort (read: looked at a lot of Wikipedia articles). No distinction is made between actual successor states and conquerors. There are a few situations I refuse to tangle with- the many tiny states in Italy and Germany/the Holy Roman Empire are prime examples.

The Roman Imperial Line (27 BCE-1453 CE):

Here's my grand unified Take. In 475, Julius Nepos (sent in 474 by Eastern Emperors Leo I and Zeno to depose the Burgundian-appointed Western Emperor Glycerius, who accepted Nepos' rule) was deposed by Orestes who put his son Romulus Augustus on the throne. Nepos fled to Dalmatia, and ruled as Emperor (recognized by the Eastern Empire) there until his assassination in 480. Syagrius, ruler of Gallo-Roman Soissons, did not accept Nepos and ruled Soissons as if it was a Roman province from 464 to 486, when his territory was conquered by Frankish king Clovis I. In 476, Romulus Augustus was overthrown by Odoacer- who was recognized as ruler of Italy by the Eastern Empire). Odoacer took over Dalmatia in 482, but the Ostrogoths- their king Theodoric also recognized as a Roman ruler by the Eastern Empire- conquered his domain in 493 and ruled it until the Eastern Empire took it in 553. In Africa, the Mauro-Roman Kingdom was effectively independent from 439 onwards. By 477, it had conquered swaths of the Vandal Kingdom and its kings claimed the title of Imperator (as did at least one king of the Kingdom of the Aurès. It allied with the Eastern Roman Empire to fight the Vandals in 535, and was largely conquered by the Eastern Roman Empire by 578. Successor states to the Mauro-Roman Kingdom and Vandal Kingdom persisted until 735, when their conquest by the Umayyad Caliphate was completed. This left the Eastern Roman Empire (which controlled both Ravenna and Rome until the 750s) as the only main Roman successor aside from the Papacy itself until the crowning of Charlemagne as Roman Emperor.

When Charlemagne was crowned Roman Emperor in 800, this began a long fourth Roman line in the from of his Carolingian Empire, which was divided in 843 into three parts. East Francia, under Otto I, generally claimed the most important portion of this line by establishing his Holy Roman Empire out of quite a few smaller states. The Holy Roman Empire, though generally not all that unified, maintained some semblance of existence for about a thousand years until Napoleon dissolved it in 1806.

The Eastern Roman Empire continued for its own thousand-year reign, interrupted in the 1200s by the Fourth Crusade but eventually refounded. Its remnant states were largely conquered by the ascendant Ottoman Empire, which claimed its title for some time.

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