Stirrups were introduced to western Europe in the timeframe of the post-Western Roman Empire and the Attila & Hun invasions of the 400s. The difference of the ability between a warrior with stirrups is an amazing factor. A fun alt-hist 3 novel series on this can be found on Amazon, “Cato’s Calvary” by Marc Jones.
Huns = Step Peoples
The Persians taught the Byzantines how to create Cataphracts (Heavy armored cavalry armed with lances)
I remember reading that stirrups are one of the reasons the Hun invasions were so successful.
On horseback and at speed, they could launch arrows almost non-stop in a standing posture. They were very adept at that and it represented a lethal advance in warfare.