I really like the Byzantine Empire.
A striking thing about its history is how important one man, the emperor, was (like Trump today, which is scary to be so dependent on one man!).
In 1025 at the death of the great emperor Basil II, the empire was at a peak of power, but only 46 years later, after a string of bad emperors, it suffered the crushing defeat of Manzikert and lost most of Anatolia.
Then three good Comneni emperors in the 12th century restored a lot of the empire’s lost power, though not as much as under Basil II, and half of Anatolia still under the Turks.
I also think some of the fluctuations in the empire’s condition was due to fluctuations in population and the health of the population, perhaps due to long-term weather fluctuations and plagues, but I haven’t studied it enough to be able to document it.
The plague during Justinians reign greatly damaged Byzantium.