Heretics and northern barbarians fellating each other with titles is hardly a large-scale macrohistorical event.
Who said the 'datestamp' itself had to be a traumatic or wonderful event to the world ? When Constantinople fell it was little more than a glorified Venetian-Genose trading post. But it effectively
marks the end of an era, just as the official end of the empty shell of a Roman Empire does, just as the accession of a semi-literate barbarian as first Holy Roman Emperor over a disorganized collection of fiefdoms marks the beginning of another. It's easy to minimize the event itself, but a judicious choice coincides with other trends. 1455 happens to coincide with the end of the Hundred Years War, when feudal cavalry no longer dominated the battlefield, after the bubonic plagues, and the beginning of the bourgeosie.
What Dachs said.
The problem with periodisation is that it's very difficult to make distinct demarcations between any two "eras" for something so general like "history" or even "European history". After all, what did Charlemagne have to do with England? Or in Poland? Aachen was peanuts compared to what they had in Byzantium.
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My argument is setting such arbitrary boundaries can be slightly dangerous and as historians, one must always be mindful that these boundaries aren't holy borders.
I appreciate the point you make, others including myself have said as much. We can mark it c. 800 or circa 850 or whatever. We know the pace of development in some areas of Europe wasn't the same as in the south, but it seems like a lot of trouble to be so wary against the dangers of setting boundaries. It's easy to be imprecise and vague, but are three three sufficiently distinct eras in European medieval history ? If we are going to bother defining them, then scholars might as well agree on a convenient frame of reference. I disagree with the 'official' breakdown by centuries: 500-1000, 1000-1300, 1300-1500.
The 'Dark Age' was a period of great instability, chaos, and few historical records for much of Europe, during which migrating tribal societies established kingdoms on the ruins of an empire thay had effectively obliterated. Those that would survive this period entrenched their language and became the foundation of many modern European states.
After Charlemagne's accession his empire stretched from Croatia to Catalonia, a large part of Christian Europe that survived the onslaught of Muslim Arabs, pagan Vikings, Avars, Magyars etc. It also confirmed the power and influence of the Catholic Church in western Europe, and marks the transition of tribal societies to feudalism. The trend was echoed in England with Egbert and Alfred the Great around the same time, whose Christian kingdom was preserved by the time it changed hands to Canute and William I. Before that, Bede is one of the only lights illuminating England's Dark Age. It was also around this time that the Byzantines took the offensive against the Abassids, and northern Spain held on to begin the reconquista. Arbitrary or not, it is sufficiently distinct from the previous era and the next.