There is a fascinating through line that runs through the history of ancient empires that, to my mind, remains an unexplored area of history, namely, the impact of the Asian horse peoples on the ancient world.
A loose collection of different groups through the centuries (i.e. Scythians, Massagetae, Issedones, Cimmerians), the nomadic peoples of central Asia made an enormous impact on the shape of history in the ancient world. In addition to their cultural and social contributions, the movement of horse peoples impacted the Assyrians, Medes, Persians, Greeks, and Romans.
The nomadic steppe people helped usher in the end of the Assyrian Empire.
They (arguably) drew Persia into further conflict with Greek Anatolian states by provoking Persians leaders to try and establish control of the Black Sea.
They profoundly influenced the cultures and possibly governance structures of the Medes and Persians, which in turn influenced the administrative structure of the Alexandrian empire.
And, not for nothing, the Huns made an enormous impact on the Roman empire, facilitating challenges and conflicts especially for the Western Roman empire in the 5th century.
Also, they possibly invented pants.
For my money, the nomadic peoples of central Asia are one of the least known elements of the ancient world who played an outsized role in the shaping of entire empires. And their definitely worth a look or two if you get a chance.