I’ve seen the Holy Roman Empire and the Roman Empire and the Byzantine Empire used far too interchangeably lately. So, in the interests of historical clarity I offer this Roman chronology.
The city (and Kingdom) of Rome was founded in 753 BC.
The Roman Republic, with Rome as its capital was created in 510 BC, after the last of the Roman kings, Tarquinius Superbus, was expelled. (No Roman king is to be confused with Napoleon Bonaparte’s son by an Austrian wife, who was also known as the King of Rome.)
The dates for the Roman Empire’s beginning are disputed.
One version has it upon Julius Caesar’s assumption of dictatorial powers for life in 44 BC.
The other version, which I am more inclined to accept is the beginning of the Augustan Principiate in 27 BC.
In the late 3rd century the Emperor Diocletian divided the empire into two halves: the West and the East. The Eastern half of the empire would last until 1453 AD. The Western would collapse in 467 AD.
In May of 330 the new capital of the Roman empire, Constantinople was consecrated by Constantine the Great. This date is also shorthand with the beginnings of the Byzantine empire.
In 380 AD the Emperor Theodosius I made Christianity the official religion of the Roman Empire, East and West.
In 467 AD the Western Empire collapsed when Odoacer forced the emperor Romulus Augustulus to abdicate.
In 800 AD Charlemagne usurped the term Roman Emperor of the West upon his coronation by Pope Leo III. This date is also shorthand for the creation of the Holy Roman Empire. The Holy Roman Empire, however, was neither holy, nor Roman, nor an empire, as Voltaire said. The Roman Empire had not existed for 333 years. Leo III gave Charlemagne the title in defiance of the rightful Roman emperor whose seat was in Constantinople. The Pope and the Patriarch and Emperor were embroiled in doctrinal disputes. Leo III gave the title to Charlemagne as a way of tweaking the nose of the emperor in Constantinople. The Holy Roman Empire was a German empire, which many consider the First Reich. The Second, of course being the Hohenzollern dynasty ruling a German empire that Bismarck created. We all know what the third was.
In 1453 the Ottomans captured Constantinople. This date is the accepted date for the final fall of the Roman empire.
It is very important to not conflate the Holy Roman empire with the Byzantine Empire. They were separate and vastly different entities.